Sewerage and Sewage Treatment

by Harold E. Babbitt

Design of a Storm Water Sewer System

50. Planning the System.—Storm sewer systems are seldom as extensive as separate or combined sewer systems, since storm sewage can be discharged into the nearest suitable point in a flowing stream or other drainage channel, whereas dry weather or combined sewage must be conducted to some point where its discharge will be inoffensive. The need of a comprehensive general plan of a storm sewer system is quite as great, however, as for a separate system. The haphazard construction of sewers at the points most needed for the moment results in the duplication of forgotten drains, expense in increasing the capacity of inadequate sewers, and difficult construction due to underground structures thoughtlessly located. A comprehensive plan permits the construction of sewers where they are needed as they are required, and enables all probable future needs to be cared for at a minimum of expense.

The same preliminary survey, map, and underground information are necessary for the design of a storm sewer system as for a separate sewer system. The map shown on Fig. 25 has been used for the design of a storm-water sewer system.

The steps in the design of a storm-water sewer system are:

1st. Note the most advantageous points to locate the inlets and lay out the system to drain these inlets. 2nd. Determine the required capacity of the sewers by a study of the run-off from the different drainage areas. 3rd. Draw the profile and compute the diameter and slope of the pipes required.

51. Location of Street Inlets.—The location of storm sewers is determined mainly by the desirable location of the street inlets. The inlets must therefore be located before the system can be planned. In general the inlets should be located so that no water will flow across a street or sidewalk, in order to reach the sewer. This requires that inlets be placed on the high corners at street intersections, in depressions between street intersections, and at sufficiently frequent intervals that the gutters may not be overloaded. City blocks are seldom so long as to necessitate the location of inlets between crossings solely on account of inadequate gutter capacity. The capacity of a gutter can be computed approximately by the application of Kutter’s formula. Inlet capacities are discussed in Chapter VI. When the area drained is sufficiently large to tax the capacity of the gutter or inlet, an inlet should be installed regardless of the location of the street intersections.

The street inlets are located on the map as shown in Fig. 25. The sewer lines are then located so as to make the length of pipe to pass near to all inlets a minimum. Storm sewers are seldom placed near the center of a street because of the frequent crowded condition on this line.

52. Drainage Areas.—The outline of a drainage area is drawn so that all water falling within the area outlined will enter the same inlet, and water falling on any point beyond the outline will enter some other inlet. This requires that the outline follow true drainage lines rather than the artificial land divisions used in locating the drainage lines in the design of sanitary sewers. The drainage lines are determined by pavement slopes, location of downspouts, paved or unpaved yards, grading of lawns and the many other features of the natural drainage which are altered by the building up of a city. The location of the drainage lines is fixed as the result of a study of local conditions.

The watershed or drainage lines are shown on Fig. 25 by means of dot and dash lines. A drainage line passes down the middle of each street because the crown of the street throws the water to either side and directs it to different inlets. A watershed line is drawn about 50 feet west of such streets as Kentucky St., Florida St., etc., because the downspouts from the houses on those streets discharge or will discharge into the street on which they face. The location of any watershed line within 20 feet more or less is, in most cases, a matter of judgment rather than exactness. Each area is given an identifying number or mark which is useful only in design. It usually corresponds to the inlet number.

53. Computation of Flood Flow by McMath Formula.—McMath’s Formula is used as an example of the method pursued when an empirical formula is adopted for the computation of run-off, and because of its frequent use in practice. Other formulas may be more satisfactory under favorable conditions.

Computations should be kept in order by a tabulation such as is shown in Table 21, in which the quantity of storm flow discharged from the sewer at the foot of Tennessee St., on Fig. 25, has been computed by means of the McMath Formula, using the constants suggested for St. Louis conditions, i = 2.75, and c = 0.75. The solutions of the formula have been made by means of Fig. 11. The column headings in the Table are explanatory of the figures as recorded. The computation should begin at the upper end of a lateral, proceed to the first junction and then return to the head of another lateral tributary to this junction. They should be continued in the same manner until all tributary areas have been covered. Special computations will be necessary for the determination of the maximum quantity of storm water entering each inlet to avoid the flooding of an inlet or gutter. These computations have not been shown as they are so easily made by the application of McMath’s Formula to each area concerned.

The determination of the average slope ratio is a matter of judgment, based on the average natural slope of the surface of the ground and an estimate of the probable future conditions.

54. Computation of Flood Flow by Rational Method.—The rational method for the computation of storm-water run-off is described in Chapter III. An example of its application to storm sewer design is given here for the district shown in Fig. 25.[^[34]] The computations are shown in Table 21. As in the preceding designs the table has been filled in from left to right and line by line. Computations have started at the upper end of laterals tributary to each junction. The column headed I represents the imperviousness factor in the expression Q = AIR. It is based on judgment guided by the constants given in Chapter III concerning imperviousness. The column headed “Equivalent 100 per cent I acres” is the product of the two preceding columns. It reduces all areas to the same terms so that they can be added for entry in the column headed “Total 100 per cent I acres.” It may be necessary to record the values for this column on several lines where the imperviousnesses of the tributary areas are different. This condition is illustrated in the last line of the table, for the length of sewer nearest the outlet. In the preceding lines the imperviousness recorded represents an average for all the tributary areas.

TABLE 21
Computations for the Quantity of Storm Sewage at the Foot of Tennessee Street on Figure 25
On Street	From Street	To Street	Identifying Number of Acres Drained	By McMath’s Formula				By Rational Method											Line Number
				Additional Acres Drained	Total Acres Drained	Slope of Surface	Run Off in C.F.S.	Area, Acres	I	Equivalent 100 Per Cent I Acres	Total 100 Per Cent I Acres	Time of Concentration, Minutes	R	Q	S	V	Sewer Length, Feet	Time in Sewer
State	N. Carolina	S. Carolina	91 and 92	2.35	2.35	0.005	5.5	2.35	0.50	1.17	1.17	7.0	4.8	5.6	0.011	4.6	300	1.1	1
State	S. Carolina	Georgia	88, 89 and 90	3.0	5.35	.005	10.8	3.00	.50	1.50	2.67	8.1	4.6	12.2	.010	5.5	300	0.9	2
State	Georgia	Florida	85, 86 and 87	3.0	8.35	.007	16.5	3.00	.50	1.50	4.17	9.0	4.4	18.3	.012	5.8	300	0.9	3
State	Florida	Kentucky	81, 83 and 84	3.0	11.35	.009	22.0	3.00	.50	1.50	5.67	9.9	4.2	23.9	.009	6.0	300	0.8	4
State	Kentucky	Tennessee	79, 80 and 82	3.0	14.35	.010	28.0	3.00	.50	1.50	7.17	10.7	4.1	29.3	.009	6.2	300	0.8	5
State	Texas	Louisiana	76 and others	3.8	3.8	.005	8.3	3.80	.35	1.33	1.33	10.0	4.2	5.6	.009	3.2	370	1.9	6
State	Louisiana	Alabama	73, 74 and 75	3.7	7.5	.007	15.0	3.70	.40	1.48	2.81	11.9	3.9	11.0	.011	5.2	300	1.0	7
State	Alabama	Tennessee	70, 71 and 72	3.0	10.5	.006	19.0	3.00	.45	1.35	4.16	12.9	3.8	15.8	.002	3.2	300	1.6	8
Tennessee	State	Talon	68, 69, 77 and 78	4.3	29.15	.15	52	4.30	.50	2.15	13.48	14.5	3.6	48.5	.019	9.8	450	0.8	9
Talon	Albemarle	Tennessee	65, 66 and 67	2.8	2.8	.018	8.4	2.80	.40	1.12	1.12	8.0	4.6	5.2	.004	3.0	210	1.2	10
Tennessee	Talon	Burnside	64 and 64a	0.7	29.85	.15	55	0.70	.20	0.14	14.74	15.3	3.5	51.5	.006	5.0	120	0.4	11
Burnside	N. Carolina	S. Carolina	57, 58 and 59	2.84	2.84	.008	7.2	2.84	.55	1.56	1.56	10.0	4.2	6.5	.008	4.5	300	1.1	12
Burnside	S. Carolina	Georgia	54, 55 and 56	3.88	6.72	.010	14.9	3.88	.55	2.13	3.69	11.1	4.0	14.8	.007	4.7	300	1.1	13
Burnside	Georgia	Florida	50, 52 and 53	3.88	10.60	.012	22	3.88	.55	2.13	5.82	12.2	3.9	22.7	.011	5.8	300	0.9	14
Burnside	Florida	Kentucky	47, 48 and 51	3.88	14.48	.013	30	3.88	.55	2.13	7.95	13.1	3.7	29.4	.016	7.5	300	0.7	15
Burnside	Kentucky	Tennessee	44, 45 and 46	3.88	18.36	.013	36	3.88	.55	2.13	10.08	13.8	3.7	37.3	.019	9.2	300	0.5	16
Tennessee	Burnside	Elm	42 and 43	2.84	51.05	.015	82	2.84	.45	2.28	26.10	15.7	3.4	88.8	.015	10.2	280	0.5	17
Elm	Above Chetwood	Chetwood	Included in next line below																18
Elm	Chetwood	Albemarle	31, 32 and 33	2.75	2.75	.007	7.0	2.75	.40	1.10	1.10	8.0	4.6	5.1	.020	5.3	480	1.5	19
Elm	Albemarle	Tennessee	27, 28, 29 and 30	5.75	8.50	.016	20	5.75	.45	2.59	3.69	9.5	4.3	15.8	.012	6.1	410	1.1	20
Tennessee	Elm	Varennes	25, 26 and 41	2.62	62.17	.017	100	2.62	.50	1.31	30.00	16.2	3.4	102	.012	10.2	180	0.3	21
Varennes	S. Carolina	Georgia	17, 18 and 19	3.17	3.17	.010	8.3	3.17	.55	1.74	1.74	9.0	4.4	7.7	.012	5.2	270	0.9	22
Varennes	Georgia	Florida	14, 15 and 16	3.17	6.34	.011	14.5	3.17	.55	1.74	3.48	9.9	4.2	14.5	.010	5.7	300	0.9	23
Varennes	Florida	Kentucky	11, 12 and 13	3.17	9.51	.013	21	3.17	.55	1.74	5.22	10.8	4.1	21.4	.017	7.7	300	0.6	24
Varennes	Kentucky	Tennessee	8, 9 and 10	3.17	12.68	.013	26	3.17	.55	1.74	6.96	11.4	4.0	27.8	.015	7.8	300	0.6	25
Tennessee	Varennes	Boulevard	6 and 7	2.32	77.17	.017	120	2.32	.55	1.28	32.84	16.5	3.3	108	.012	10.2	230	0.4	26
Tennessee	Boulevard	Outlet	1, 2, 3, 4, and 5	4.72	81.89	.017	122	0.18	.80	0.14	Area No. 1								27
								1.38	.50	0.69	Area No. 2								28
								2.80	.55	1.54	Areas No. 3 and 4								29
								0.36	.75	0.27	35.48	16.9	3.3	117	Areas No. 1–5 inclusive				30

The time of concentration in minutes is assumed by judgment for the first area. For all subsequent areas it is the sum of the time of concentration for the area or areas tributary to the inlet next above and the time of flow in the sewer from the inlet next above to the inlet in question. For example, in line 2 the time 8.1 minutes is the sum of 7.0 minutes time of concentration to the inlet at the corner of State and North Carolina St., and the time of flow of 1.1 minute in the sewer on State St. from North Carolina St. to South Carolina St. Where two sewers are converging as at the corner of Varennes Road and Tennessee St. the longest time is taken. For example, the time of concentration down Varennes Road to Tennessee St. is shown in line 25 as 11.4 + 0.6 = 12.0 minutes. The time to the same point down Tennessee St. is shown in line 21 as 16.2 + 0.3 = 16.5 minutes. This time is therefore used in line 26.

R, the rate of rainfall in inches per hour is determined by Talbot’s formula.

Q, is in cubic feet per second and is the product of the 8th and 10th columns. Since the 8th column is the sum of the products of the 5th and the 6th columns for the lines representing tributary areas, then the 11th column is the product of A, I, and R.

S, is the slope on which it is assumed that the sewer will be laid. It is usually assumed as parallel to the ground surface unless the velocity for this slope becomes less than 2 feet per second. In such a case the slope is taken as one which will cause this velocity.

V, the velocity in feet per second, is computed from diagrams for the solution of Kutter’s formula. The length in feet is scaled from the map as the distance between inlets or groups of inlets, and the time is the length in feet divided by the velocity in feet per minute.

Having computed the quantity of flow to be carried in the sewer, the design is completed by drawing the profile and computing the diameters and slopes by the same method as used in the design of separate sewers.

CHAPTER VI
APPURTENANCES

55. General.—The appurtenances to a sewerage system are those devices which, in addition to the pipes and conduits, are essential to or are of assistance in the operation of the system. Under this heading are included such structures and devices as: manholes, lampholes, flush-tanks, catch-basins, street inlets, regulators, siphons, junctions, outlets, grease traps, foundations and underdrains.

56. Manholes.—A manhole is an opening constructed in a sewer, of sufficient size to permit a man to gain access to the sewer. Manholes are the most common appurtenances to sewerage systems and are used to permit inspection and the removal of obstructions from the pipes. The details of the Baltimore standard manholes are shown in Fig. 27 and a manhole on a large sewer in Omaha is shown in Fig. 28. The features of these designs which should be noted are the size of the opening and working space, and the strength of the structure. Manhole openings are seldom made less than 20 inches in diameter and openings 24 inches in diameter are preferable. A man can pass through any opening that he can get his hips through provided he can bend his knees and twist his shoulders immediately on passing the hole. For this reason the manhole should widen out rapidly immediately below the opening, as shown in Fig. 27 and 38.

The walls of the manhole may be built either of brick or of concrete. Brick is more commonly used, as the forms necessary for concrete make the work more expensive unless they can be used a number of times. The walls of the manhole should be at least 8 inches thick. Greater thicknesses are used in treacherous soils and for deep manholes, or to exclude moisture. A rough expression for the thickness of the walls of a brick manhole more than 12 feet deep in ordinary firm material is t = d
2 + 2, in which t is the thickness in inches and d is the depth in feet. The thickness of brick walls may be changed every 5 to 10 feet or so. Concrete walls may be built thinner than brick walls.

Fig. 27.—Baltimore Standard Manhole Details.

The bottoms of brick manholes are frequently made of concrete as shown in Fig. 27. The floor slopes towards the center and is constructed so that the sewage flows in a half round or U-shaped channel of greater capacity than the tributary sewers. The sides of the channel should be high enough to prevent the overflow of sewage onto the sloping floor, which should have a pitch of about one vertical to 10 or 12 horizontal. In manholes where two or more sewers join at approximately the same level the channels in the bottom should join with smooth easy curves. Where the inlet and outlet pipes are not of the same diameter the tops of the pipes should ordinarily be placed at the same elevation to prevent back flow in the smaller pipes when the larger pipes are flowing full.

The dimensions of the manhole should not be less than 3 feet wide by 4 feet long for a height of at least 4 feet, when built in the form of an ellipse, or 4 feet in diameter when built circular. No standard method for the reduction of the diameter of the manhole near the top is observed, the rate being more or less dependent on the depth of the manhole. The use of sloping sides above the frost line is desirable as such a form is more resistant to heaving by frost action.

For sewers up to 48 inches in diameter the manhole is usually centered over the intersection of the pipes and has a special foundation. For larger sewers the manhole walls spring from the walls of the sewer as shown in Fig. 28.

Fig. 28.—Details of a Manhole and a Well Hole.

In the case of a decided drop in the elevation of a sewer, or of a tributary sewer appreciably higher than an outlet in any manhole, the sewage is allowed to drop vertically at the manhole, hence the name drop manhole. The Baltimore standard drop manhole is shown in Fig. 27. A well hole is an unusually deep drop manhole in which the force of the vertical drop of sewage is broken by a series of baffle plates, or by a sump at the bottom of the well hole. Fig. 28 shows a well hole at St. Paul, Minn. The use of drop manholes can be avoided in large sewers by the construction of a flight of steps or flight sewer as shown in Fig. 29, which allows the use of a steep grade and serves to break the velocity of the sewage.

The specifications of the Sanitary District of Chicago, covering the construction of manhole covers and frames are:

All castings shall be of tough, close grained, gray iron, free from blow holes, shrinkage and cold shuts, and sound, smooth, clean and free from blisters and all defects.

All castings shall be made accurately to dimensions to be furnished and shall be planed where marked or where otherwise necessary to secure perfectly flat and true surfaces. Allowance shall be made in the patterns so that the specified thickness shall not be reduced.

All castings shall be thoroughly cleaned and painted before rusting begins and before leaving the shop with two coats of high grade asphaltum or any other varnish that the Engineer may direct. After the castings have been placed in a satisfactory manner, all foreign adhering substances shall be removed and the castings given one additional coat of asphaltum. No castings shall be accepted the weight of which shall be less than that due to its dimensions by more than 5 per cent.

Fig. 29.—Flight Sewer at Baltimore.
Eng. Record, Vol. 59, p. 161.

Fig. 30.—Baltimore Standard Manhole Frame and Cover.

The weights of frames and covers in use vary from 200 to 600 pounds, the weight of the frame being about 5 times that of the cover. The lightest weights are used where no traffic other than an occasional pedestrian will pass over the manhole. Frames and covers weighing about 400 pounds are commonly used on residential streets, whereas 600 pound frames and covers are desirable in streets on which the traffic is heavy. The frames should be so designed that the pavement will rest firmly against it and wear at the same rate as the surrounding street surface. Experience has shown that vertical sides should be used for the outside of the frame to approach this condition, and that the frame should not be less than 8 inches high. The cover should be roughened in some desirable pattern as shown in Fig. 30. Smooth covers become dangerously slippery. Where the ventilation of the sewers is not satisfactory the manhole covers are sometimes perforated. This is undesirable from other points of view as the rising odors and vapors are obnoxious at the surface and the entering dirt and water are detrimental to the operation of the sewer. The stealing and destruction of manhole covers and the unauthorized entering of sewers has occasionally required the locking of the covers to the frame when in place. The locks commonly used consist of a tumbler which falls into place when the manhole is closed, and which can be opened only by a special wrench or hook. Adjustable frames are sometimes used where the street grade is settling, or may be raised in order that the elevation of the top of the cover may be made to conform to that of the street surface, without reconstructing the top of the manhole. One type of adjustable cover is shown in Fig. 31. Manhole covers should be so marked that the sanitary sewer can be distinguished from the storm-water sewer, and both from the telephone conduit, etc.

Fig. 31.—Adjustable Manhole Frame and Cover.

Iron steps are set into the walls of the manhole about 15 inches apart vertically to allow entrance and exit to and from the manhole. Galvanized iron is preferable to unprotected metal as the action of rust is particularly rapid in the moist air of the sewer. One type of these manhole steps is shown in Fig. 27. The steps should have a firm grip in the wall as a loose step is a source of danger.

Fig. 32.—Baltimore Standard Lamphole.

57. Lampholes.—A lamphole is an opening from the surface of the ground into a sewer, large enough to permit the lowering of a lantern into the sewer. Lampholes are used in the place of manholes to permit the inspection or the flushing of sewers, and to avoid the expense of a manhole. They are located from 300 to 400 feet from the nearest manhole in such a manner that a lamp lowered in the lamp hole can be seen from the two nearest manholes.

Lampholes should be constructed of 8– to 12–inch tile or cast-iron pipe. The lower section should be a cast-iron T on a firm foundation, but if constructed of tile it should be reinforced with concrete to take up the weight of the shaft. The details of the Baltimore standard lamphole are shown in Fig. 32. Lampholes are not commonly used on sewerage systems on account of their lack of real usefulness and the troubles encountered by breaking of the pipe below the shaft.

58. Street Inlets.—A street inlet is an opening in the gutter through which storm water gains access to the sewer. The types used in different cities vary widely. Details of an inlet in successful use are shown in Fig. 33. The figure shows also a common form of connection to the sewer. A water-seal trap is sometimes used to prevent the escape of odors from the sewer. Care must be taken in design that such traps do not freeze in winter nor dry out in summer, although it is not always possible to prevent these contingencies.

Fig. 33.—Details of an Untrapped Street Inlet, without Catch-Basin.

The important features to be observed in the design of a street inlet are: height and length of opening, character of grating, and location. The general location of inlets is discussed in Chapter V. The clear height of opening commonly used is from 5 to 6 inches, with a clear length of 24 to 30 inches or longer. Inlets of this size have given satisfaction on paved streets with moderate slopes, where the drainage area is not greater than 10,000 to 12,000 square feet of pavement. W. W. Horner states:[^[35]]

The St. Louis type of inlet “old” style was a vertical opening in the curb 8 inches high and 4 feet in length with a horizontal bar making the net opening about 5 inches. It has a broad sill extending under the sidewalk. The “new” style inlet is 4½ feet long with a clear opening of 6 inches and no bar. The sill is done away with and the opening drops down directly from the curb. Tests were made of the capacity of this inlet on pavements on different slopes with sumps of depths varying from 0 to 6 inches in front of the inlet, extending out 3 feet from the gutter, and returning to the elevation of the gutter at a slope of 3 inches to the foot. The results of these tests are shown in Table 22. The capacity of the inlet is expressed as the amount of water entering just before some water begins to lap past. If a large amount of water is allowed to flow past much more water will enter the inlet thus furnishing a factor of safety for large storms. It was noted that by beginning the rise in the pavement about opposite the middle of the inlet the capacity of the inlet was increased from 20 to 50 per cent.

TABLE 22
Capacities of St. Louis Street Inlets
From tests by W. W. Horner. Cubic feet per second
Slope in Per Ct.	0.42				1.5				2.85				4.5
Depth of Sump, Inches	0.0	2	4	6	0	2	4	6	0	2	4	6	0	2	4	6
Capacity, old style			1.27						0.03	0.25	0.78	1.49
Capacity, new style	0.1	0.5	1.5	2.5	0.08	0.4	1.1	2.1	0.03	0.28	0.87	1.62	0.02	0.15	0.45	1.0

Gratings with horizontal bars will admit more water than gratings with vertical bars, but they will also admit more rubbish such as sticks, papers, leaves, etc., which serve to clog the sewers. Vertical barred gratings and gratings in the bottom of the gutter clog more quickly than other types. In the selection of the type of grating to be used a decision must be made as to whether it is more desirable to clean the sewer or catch-basin, or to flood the street as a result of clogged inlets. Where catch-basins are used or the sewers are large, horizontal bars are more satisfactory. The openings between bars should be small enough to prevent the entrance of a horse’s hoof or objects of sufficient size to clog the sewer. Four inches in the clear for vertical openings and 6 inches for horizontal openings are reasonable sizes.

The location of the inlets at the intersection of the two curb lines at a corner results in a lower first cost but on heavily traveled streets this may result in a higher maintenance cost than for other locations because of the concentration of traffic at street corners, hammering the inlet casting out of shape or position. Vehicles making short turns will tend to climb the curb and will intensify the wear upon the inlet. These objections can be overcome by the use of two inlets at each corner, set back far enough from the curb intersection to avoid interference with the cross-walks. This also makes it possible to raise the cross-walks without the use of gutters under them.

The size of the pipe from the inlet to the catch-basin or sewer should be large enough to care for all of the water which may enter the inlet. As the capacity of the inlet is seldom known with accuracy and the capacity of the pipe is difficult of determination, it has become customary to use a 10–inch or a 12–inch connecting pipe for each ordinary independent inlet.

59. Catch-basins.—Catch-basins are used to interrupt the velocity of sewage before entering the sewer, causing the deposition of suspended grit and sludge and the detention of floating rubbish which might enter and clog the sewer. A separate catch-basin may be used for each inlet, or to save expense, the pipes from several inlets may discharge into one catch-basin.

Fig. 34.—Catch-basin.
Outlets are not always trapped.

The types in successful use are extremely varied, but the distinguishing feature of all is an outlet located above the floor of the basin. A common form of catch-basin is shown in Fig. 34. It is constructed similar to a manhole with a diameter of about 4 or 4½ feet and a depth of retained water from 3 to 4 feet. Catch-basins of this size will care for the sewage from the inlets at the four corners of a street intersection, each draining a city block. In unusual situations it may be necessary to install a larger basin, but too large a catch-basin is less desirable than one which is too small, as the former stinks and the latter is useless. Traps are sometimes used to prevent the escape of odors from the sewer into the street, but odors are often created in the catch-basins themselves. Some engineers arrange the trap so that it can be opened for observation down the sewer as in Fig. 34, thus combining the advantages of a manhole with the catch-basin.

The use of catch-basins is objectionable because: they furnish a breeding place for mosquitoes and other flying insects; the septic action in them produces offensive odors; if on a combined sewer they permit the escape of offensive odors in dry weather when the water seal in the trap has evaporated; and the freezing of the water seal in the trap prevents the entrance of water to the sewer. The sole advantage lies in the prevention of the clogging of the sewers, but this may be sufficient to overbalance all of the disadvantages. In general catch-basins should be provided on paved streets which are cleaned by flushing the material into the sewers, or where the drainage is from an unimproved or macadamized street, sandy country, or into sewers in which the velocity of flow is less than 2 feet per second.

Fig. 35.—Diagrammatic Section through a Grease Trap.

60. Grease Traps.—The presence of grease in sewers results in the formation of incrustations which are difficult to remove and which cause a material loss in the capacity of the sewer. The presence of oil and gasoline has resulted in violent and destructive explosions as is described in Chapter XII. A type of grease trap used on the drains from hotels, restaurants, or other large grease producing industries is shown in Fig. 35. The trap is similar to a catch-basin except that it is too small for a man to enter, and the outlet is necessarily trapped in order to prevent the escape of grease. The details of a gasoline and oil separator approved by the New York City Fire Department are shown in Fig. 36.[^[36]]

Fig. 36.—Gasoline and Oil Separator.

61. Flush-Tanks.—These are devices to hold water used in flushing sewers. They are required only on sanitary and combined sewers. Their use tends to prevent the clogging of sewers laid on flat grades and permits flatter grades in construction than could otherwise be adopted. Flush-tanks may be operated either by hand or automatically. Automatic operation is more common than hand operation. The hand-operated tanks are similar to manholes so arranged that the inlet and outlet sewers can be plugged while the manhole or tank is being filled with water either from a hose or a special service connection. When sufficient water has been accumulated the outlet is opened and the sewer is flushed by the rush of water. A sluice gate, flap valve, or a specially fitted board is sufficient to fit over the mouth of the inlet and outlet during the filling of the tank. Such an arrangement has the advantage of being cheap, simple, and satisfactory, though somewhat crude. In some cases water is run into the tank at the same rate that it is discharged through the open outlet, maintaining a depth of 4 or 5 feet in the tank until the water passing the manhole below runs clean. The volume of water required by this method is large. Flushing water under a relatively high head is sometimes obtained by the use of tank wagons which are quickly emptied into the sewer through a canvas pipe dropped down a manhole. In all such cases if not well constructed the manhole is subject to caving due to the rush of water around the outlet. Precautions should be taken to minimize this danger by limiting the depth of water which may be accumulated. This can be done by constructing an overflow at a height of 4 or 5 feet above the bottom of the manhole, discharging into the sewer through an outside drain.

Automatic flush-tanks are constructed similar to a manhole, but special care should be taken to make them water-tight. The apparatus for providing the automatic discharge may operate either with or without moving parts, the latter being preferable as they require less attention and are not so liable to get out of order. An automatic flush-tank of the Miller type is shown in Fig. 37. It is a patented device manufactured by the Pacific Flush Tank Company. The small pipe at the left is a service connection to the water main. Water is allowed to flow continuously into the tank at such a rate as to fill it in the required interval between discharges. The tanks are discharged as nearly once a day as it is practicable to regulate them. The rate of flow into the tank is determined by trial and varies to some extent with the water pressure. The regulator shown in the figure is desirable as the continuous flow through the ordinary cock soon wears it away. Some waters will cause deposits to form in the small passages of the cocks or regulators, thus cutting off the flow.

Fig. 37.—Automatic Flush-Tank.
Pacific Flush Tank Co.

The tank operates as follows: when the water rising in the tank reaches the bottom of the bell, air is trapped in the bell and prevented from escaping through the main trap by the water at A. As the water continues to rise in the tank the air in the bell is compressed, the water level at A is driven down and water trickles from the siphon at C. The height of the water in the tank above the level of the water in the bell is equal at all times to the height of C above the lowering position of A. When A reaches the position of B a small amount of air is released through the short leg of the trap and a corresponding volume of water enters the bell. The head of water above the bell then becomes greater than the head of water in the short leg of the trap, which results in the discharge of all of the air in the long leg of the trap and the rapid discharge of the water in the tank through the siphon. The discharge is continued until the siphonic action is broken by the admission of air when the water level in the tank is lowered to the bottom of the bell. The size of the siphons is fixed by the diameter of the leg of the siphon. Table 23 shows the capacity and size of sewers for which the different sizes of siphons are recommended by the manufacturers.[^[37]]

TABLE 23
Sizes of Siphons to be Used with Automatic Flush-Tanks
Diameter of Siphon in Inches	Diameter of Tank at the Discharge Line in Feet	Total Discharge for One Flush in Gallons	Average Rate of Discharge in Sec.-ft.	Diameter of Sewer in Inches	Height of the Discharge Line above the Edge of the Bell
4	3	60	0.35	4 to 6	1 ft. 2 in.
5	3	100	0.73	6 to 8	1 ft. 11 in.
6	4	240	1.06	8 to 10	2 ft. 6 in.
8	4	280	2.12	12 to 15	2 ft. 11 in.

When flush-tanks are placed at the upper end of laterals provision should be made for inspecting and cleaning the sewer by the construction of a separate manhole, or by combining the features of a manhole and a flush-tank in the same structure. Such a combination is shown in Fig. 38 from a design by Alexander Potter.

Except under unusual conditions flush-tanks are used only on separate sewers. They should be placed at the upper end of laterals in which the velocity of flow when full is less than 2 to 4 feet per second. The capacity of the tank or the volume of the dose is dependent on the diameter and slope of the sewer. The most effective flush is obtained by a volume of water traveling at a high velocity and completely filling the sewer. A large volume allowed to run slowly through the sewer will have but little if any flushing action. Data on the quantity of flushing water needed are given in Table 24.[^[38]] As the result of a series of experiments conducted by Prof. H. N. Ogden on the flushing of sewers,[^[39]] the conclusion was reached that the effect of a flush of about 300 gallons in an 8–inch sewer on a grade less than 1 per cent would not be effective beyond 800 to 1,000 feet, but that on steeper grades much smaller quantities of water would produce equally good results.

Fig. 38.—Automatic Flush-Tank and Manhole.
Miller-Potter Design. Pacific Flush Tank Co.

TABLE 24
Gallons of Water Needed for Flushing Sewers
Slope	Diameter of Sewer in Inches
	8	10	12
0.005	80	90	100
.0075	55	65	80
.01	45	55	70
.02	20	30	35
.03	15	20	24

Engineers do not agree upon the advisability of the use of automatic flush-tanks, some believing that they are a needless expense that can be avoided by hand flushing, and others feeling that a flush-tank should be placed at the upper end of every lateral. These diverse opinions are the result of different experiences in different cities.

62. Siphons.—There are two forms of siphons used in sewerage practice, a true siphon and an inverted siphon. A true siphon is a bent tube through which liquid will flow at a pressure less than atmospheric, first upwards and then downwards, entering and leaving at atmospheric pressure. An inverted siphon is a bent tube through which liquid will flow at a pressure greater than atmospheric first downwards and then upwards, entering and leaving at atmospheric pressure.

In sewerage practice the word siphon refers to an inverted siphon unless otherwise qualified. Siphons, both true and inverted, are used in sewerage systems to pass above or below obstacles. True siphons are seldom used as they must be kept constantly filled with liquid.[^[40]] Accumulated gas must be removed in order to prevent the breaking of the siphon which results in the cessation of flow. By the breaking of a true siphon is meant the stoppage of siphonic action due to the accumulation of air or gas at the peak of the siphon. Since the rate of flow of sewage fluctuates widely it is extremely difficult to control the flow so that a true siphon may be completely filled with liquid at all times.

In the design of inverted siphons care must be taken to prevent sedimentation, and to permit inspection and cleaning. Sedimentation is prevented by maintaining a velocity greater than a fixed minimum, usually taken at about 2 feet per second. This minimum is attained by providing a number of channels. The smallest channel is designed to convey the least expected flow at the minimum velocity. Each of the other channels is made as small as possible, within the limits of economy and simplicity, in order that the minimum velocity shall be exceeded quickly after flow has commenced in them. The last channel or channels to be filled are made somewhat larger, because the sewage conveyed in them contains less settleable matter than is contained in the more concentrated dry weather flow. The type of siphon used in New York to pass under the subway is shown in Fig. 39. Note should be taken of the clean-out manhole provided on the 14–inch pipe. The other pipes are large enough for a man to enter and clean.

Fig. 39.—Sewer Siphon under New York Subway.
Eng. News Vol. 76, p. 443.

The computations involved in the design of a siphon are illustrated in the following example, in which it is desired to construct a siphon to pass under the railway cut shown in Fig. 40. The first step is to determine the limiting diameter and slope of the smallest pipe in the siphon. One-sixth of the capacity of the 6–foot approach sewer or 19 cubic feet per second will be assumed as the minimum flow. The diameter of the pipe necessary to carry 19 cubic feet per second at a velocity of 2 feet per second is 42 inches. The hydraulic gradient should have a slope of 0.0005 if the material used has a roughness coefficient of .015. This is the minimum permissible slope of the siphon. The selection of a steeper slope will necessitate the laying of the sewer at a greater depth, and will permit the use of smaller pipes for the siphon. The selection of the exact slope must then be based on judgment with the minimum limitation above placed. The slope will be arbitrarily selected as 0.001, the same as that of the approach sewer. The diameter of the dry weather pipe will therefore be 36 inches, with a capacity of 18 second-feet, which is approximately the assumed dry weather flow. The velocity of flow will be 2.5 feet per second. The length of flow along the siphon is 150 feet.

Fig. 40.—Diagram for the Design of an Inverted Siphon.

The next step should be the determination of the elevation at the lower end of the 36–inch pipe. This is done by multiplying the assumed grade by the equivalent length of straight pipe, and subtracting the product from the elevation at the upper end. The length of straight pipe which will give the same loss of head as the siphon is called the equivalent pipe. It is determined as follows:

First, determine the head loss at entrance. This will vary between nothing and one velocity head, dependent on the arrangement at the entrance.[^[41]] The length of straight pipe which will give this same loss can be computed from the expression l = h
S, using for S the assumed slope of the hydraulic gradient.

Second, determine the head loss due to the bends, This is determined from the expression

h = fl
d V²
2g

in which h = the head loss in the bend; l = the length of the bend; d = the diameter of the pipe; v = the average velocity of flow; g = the acceleration due to gravity; f = a factor dependent on the radius (R) of the bend and d.

The relation between f, R, and d, for 90° bends is shown as follows:[^[42]]

R d	24	16	10	6	4	2.4
f	0.036	0.037	0.047	0.060	0.062	0.072

After the head loss has been determined, the equivalent length of straight pipe is determined as above.

Third. The equivalent length of pipe will be the sum of the actual length of pipe and the equivalent lengths as found above.

In the problem in hand the head lost at the entrance will be assumed as one-third of a velocity head, or 0.0324 foot. With the assumed slope of 0.001 this is equivalent to 32 feet of pipe. The radius of the bend is about 20 feet and the length for a 45° central angle is about 16 feet. The head loss for this angle will probably be a little more than one-half that for a 90° angle. The expression will therefore be taken as about 0.2V²
2g and for two bends is equivalent to about 40 feet of pipe. The equivalent length of pipe is therefore 150 + 32 + 40 = 222 feet. The elevation at the lower end should therefore be: the elevation at the upper end, 92.07 − 222 × .001 = 91.85.

The diameters of the remaining pipes in the siphon are determined so that the sewage in the approach sewer is backed up as little as is consistent with good judgment before each pipe comes into action. This is done satisfactorily by a method of cut and try. Let it be assumed that the siphon will be composed of three pipes: the dry weather pipe taking 18 second-feet, the second pipe taking 28 second-feet, and the third pipe taking the remaining 70 second-feet. The diameters of the two larger pipes on the assumed slope of 0.001 will therefore be 42 inches and 60 inches respectively. Other combinations might be used which would be equally satisfactory. There are many methods by which the sewage can be diverted into the different channels of the siphon. For example, the openings into the different pipes may be placed at the same elevation, and the sewage allowed to enter them in turn through automatically or hand-controlled gates, or in another method of control the openings may be placed at such elevations that when the capacity of one pipe has been exceeded the sewage will flow into the next largest pipe as shown in Fig. 40. The outlets from the different pipes are ordinarily placed at the same elevation, thus leaving each pipe standing full of sewage. Stop planks should be provided at the outlet in order that the pipes may be pumped out for cleaning. The objection to this arrangement is that the larger pipes may operate at a velocity less than 2 feet per second, and they will be standing full of sewage which might become septic. However, as they will take nothing but the storm flow near the top of the sewer no difficulty should be encountered from sedimentation in them, and all are large enough for a man to enter for inspection or cleaning.

Fig. 41.—Coffin Sewer Regulator.

63. Regulators.—Regulators are commonly used to divert the direction of flow of sewage in order to prevent the overcharging of a sewer or to regulate the flow to a treatment plant. Sewer regulators are of two types, those with moving parts and those without moving parts. An example of the moving part type is shown in Fig. 41. In this type as the sewage rises the float closes the gate to the inlet sewer, thus preventing the entrance of sewage under head from the larger sewer. There are many variations in the details of float controlled regulators, but the principle of operation is similar in all. These regulators can be adjusted to fix the maximum rate of flow to a relief channel or sewage treatment plant, or during times of storm to cut off the outlet to the dry weather channel. Another form of the moving part type is shown in Fig. 42.[^[43]] It has been used extensively by the Milwaukee Sewerage Commission. In its operation the dry weather flow is diverted by the dam into the intercepter. It passes under the movable gate on its way to the treatment plant. As the flow increases the dam is overtopped and flood waters are discharged down the storm channel. The movable gate is hung on a pivot placed below center. As the water rises in the intercepter, the pressure against the upper portion of the gate becomes greater than that against the lower portion, and the gate closes. An opening is left at the bottom to allow an amount of sewage equal to the dry weather flow to escape beneath the gate to prevent clogging or sedimentation in the intercepter channel.

Objections to all moving part regulators are their need of attention and liability to become clogged.

Fig. 42.—Moving Part Regulator without Float.

Fig. 43.—Leaping Weir at Danville, Illinois.

Fig. 44.—Overflow Weir at San Francisco.
Eng. News, Vol. 73, p. 307.

Fig. 45.—Overflow Weir in Action.
Shadow of steel knife edge which forms the lip of the weir can be seen through the falling sewage.

The overflow weir and the leaping weir have no moving parts and are used for the regulation of the flow in sewers. A leaping weir is formed by a gap in the invert of a sewer through which the dry weather flow will fall and over which a portion or all of the storm flow will leap. One form of leaping weir is shown in Fig. 43. An overflow weir is formed by an opening in the side of a sewer high enough to permit the discharge of excess flow into a relief channel. A weir at San Francisco is shown in Fig. 44. A series of tests were run on leaping weirs and overflow weirs in the hydraulic laboratory of the University of Illinois. The type of leaping weir tested was formed by the smooth spigot end of a standard vitrified sewer pipe. The overflow weirs were formed by a steel knife edge in the side of the pipe parallel to its axis as shown in Fig. 45. Tests were made in 18–inch and 24–inch pipes on various slopes from 0.018 to 0.005, for both leaping weirs and overflow weirs. The overflow weirs were varied in length from 16 inches to 42 inches and were placed at various heights from 25 per cent to 50 per cent of the diameter above the invert of the sewer. As the result of the observations the following formulas were developed. For the leaping weir the expressions for the coordinates of the curve of the surfaces of the falling stream, are:

For the outside surface x = 0.53V^⅔ + y

For the inside surface x = 0.30V

+ y^¾

in which x and y are the coordinates. The origin is in the upper surface of the stream vertically above the end of the invert of the pipe. The ordinate y is measured vertically downwards. V is the velocity of approach in feet per second. These expressions are applicable to any diameter of sewer up to 10 or 15 feet. They should not be used for depths of flow greater than about 14 inches; nor for slopes of more than 25 per 1,000; nor for velocities less than 1 foot per second nor more than 10 feet per second. The expression for the ordinate of the inside curve is not good for less than 6 inches nor more than 5 feet. The expression for the ordinate of the outside curve is limited to values between the origin and 5 feet below it.

The expression for the length of an overflow weir of the type shown in Fig. 45, necessary to discharge a given quantity, is in the form,

l = 2.3Vd log h₁
h₂

in which l = the length of the weir in feet; V = the velocity of approach in feet per second; d = the diameter of the pipe in feet; h₁ = the head of water on the upper end of the weir; h₂ = the head of water on the lower end of the weir.

In the design of an overflow weir by this formula the height of the weir above the invert of the sewer and the flow over the weir should be determined arbitrarily. The height should be subtracted from the computed depth of water above the weir to determine the value of h₁. The difference between the flow over the weir and the flow above the weir will represent the rate of flow in the sewer below the weir. The value of h₂ can then be computed as the difference in the depth of flow below the weir and the height of the weir above the invert. The value of V is computed from Kutter’s formula. The length of the weir is determined by substituting these values in the formula.

64. Junctions.—At the junction of two or more sewers the elevation of the inverts should be such that the normal flow lines are at the same elevation in all sewers. The sewers should approach the junction on a steep grade to prevent sewage backing up in one when the other is flowing full. The velocity of flow at the junction should not be decreased and turbulence should be avoided in order to prevent sedimentation and loss of head. The junction should be effected on smooth easy curves with radii at least five times the diameter of the sewer where possible. Curves with short radii cause backing up of sewage thus reducing the capacity of the sewers.

The terms bellmouth or trumpet arch are sometimes applied to the junction of sewers large enough to be entered by a man. In small sewers the Y branches and special junctions are manufactured so that the center lines of the pipes intersect, and the junctions of mains and laterals are made in manholes. In the construction of a bellmouth the arch is carried over all the sewers. A manhole should be constructed at these junctions as clogging frequently occurs there, due to swirling and back eddies which cannot be avoided.

65. Outlets.—The outlets to a sewerage system discharging into a swiftly running stream must be protected against wash and floating debris. In a stream or other body of water subject to wide variations in elevation the backing up of the sewage during high water should be avoided. Where tidal flats or low ground about the outlet may be alternately submerged and uncovered the discharge should always be into swiftly running water. In quiescent bodies of water such as lakes and harbors, and in rivers where the dilution is low, and in many other cases, the sewer outlet should be submerged.

Fig. 46.—Tide Gate.

Outlets are protected against wash and the impact of debris by the construction of deep foundations and heavy protecting walls. Although the construction of an outlet in a slow current or a back eddy would avoid danger from wash and debris, the discharge of the sewage into the most rapid current possible aids in the prevention of a local nuisance. A row of batter piles on the upstream or exposed side of the sewer is desirable, or it may be necessary to construct a break-water to prevent the wash of the current from dislodging the pipe. These break-waters are low dams of wood or broken stone, more or less loosely thrown together. The backing up of water into the sewer can be prevented by constructing the sewer above the outlet on a steep grade. Where this is not possible the use of tide gates will be helpful. A tide gate, one form of which is shown in Fig. 46, is a special form of check valve placed on the end of the sewer.

Fig. 47.—Sewer Outlet on a Trestle.
Eng. News, Vol. 49, p. 9.

Sewer outlets are sometimes constructed on long trestles in order to reach deep or running water. Such a trestle is shown in Fig. 47. In Boston the outlet sewers are submerged under the harbor and discharge through outlets well out in the tidal currents. The sewage is discharged under pressure and the pumps are operated at some of the stations only at such times as the tidal currents will carry the sewage away from the harbor. It is not always necessary in a combined sewerage system to carry the storm flow to a distant submerged outlet. A double outlet can be constructed as shown in Fig. 48 in which the dry weather flow is carried to the channel in a submerged sewer and the storm flow is discharged on the bank.[^[44]] Cast-iron pipe should be used for submerged outlets as the sewer is subject to disturbance by the currents, anchors, ice, and other causes.

Fig. 48.—Dry Weather and Storm Sewer Outlet at Minneapolis, Minnesota.
Eng. Record, Vol. 63, p. 383.

66. Foundations.—Sewers constructed in firm dry soil require no special foundation to distribute the weight over the supporting medium. In soft materials the lower half of the sewer ring may be spread as shown in Fig. 22, and in rock the pressures on sewer pipes are evenly distributed by a cushion of sand. In wet ground such as quicksand, mud, swamp land, etc., a foundation must be constructed if the water cannot be drained off.

The permissible intensities of pressure on foundations in various classes of material allowed by the building codes in different cities are given in Table 25. These figures are based on the assumption that the material is restrained laterally, which is generally the condition in sewer construction. In the softer materials it becomes necessary to spread the foundations not only to reduce the intensity of pressure, but also to care for the thrust of the sewer arch. The arch thrust may be found by one of the methods of arch analysis, and the haunches spread to care for this, or the sewer invert may be transversally reinforced to assist in caring for this action. Some sewer sections in hard and soft material are shown in Fig. 22 and 23.

TABLE 25
Allowable Bearing Value on Soils in Various Cities
From Proc. Am. Soc. Civil Engrs., Vol. 46, 1920, p. 906
Quicksand and alluvial soil	½ to 1 ton per sq. ft. for Providence, R. I., ½ ton per sq. ft. for 6 cities
Soft clay	1 ton per sq. ft. for 27 cities, ¾ ton per sq. ft. for New Orleans, 2 to 3 tons for Providence, R. I.
Moderately dry clay and fine sand, clean and dry	2 tons for 7 cities, 1¾ to 2¼ for Chicago, 2½ tons for Louisville, 2 to 4 tons for Providence, 3 tons for Grand Rapids and Los Angeles
Clay and sand in alternate layers	2 tons for 19 cities, 1¾ to 2¼ for Chicago, 3 to 5 tons for Providence
Firm and dry loam or clay, or hard dry clay or fine sand	3 tons for 24 cities, 2½ tons for 2 cities, 2 to 3 tons for Atlanta, 3½ tons for Philadelphia, 4 tons for 6 cities
Compact coarse sand, stiff gravel or natural earth	4 tons for 25 cities, 3½ tons for Buffalo, 3 to 4 tons for Atlanta, 4 to 5 tons for Cincinnati, 5 tons for Denver, 4 to 6 tons for 3 cities, 6 tons for Rochester, N. Y.
Coarse gravel, stratified stone and clay, or rock inferior to best brick masonry	6 tons for 3 cities, 5 tons for 2 cities, 8 tons for 1 city
Gravel and sand well cemented	8 tons for 5 cities, 6 tons for 2 cities, 8 to 10 tons for 1 city
Good hard pan or hard shale	10 tons for 4 cities, 6 tons for 2 cities, 8 tons for 1 city
Good hard pan or hard shale unexposed to air, frost or water	8 tons for 1 city, 10 to 15 tons for 1 city, 12 to 18 tons for 1 city
Very hard native bed rock	20 tons for 5 cities, 15 tons for 1 city, 10 tons for 1 city, 25 to 50 tons for 1 city
Rock under caisson	24 tons for Baltimore, 25 tons for Cleveland

On soft foundations such as swamps or for outfalls on the muck bottom of rivers the sewer may be carried on a platform. For small sewers 2–inch planks, 2 to 4 feet longer than the diameter of the pipe are laid across the trench, and the sewer rests directly upon them. For large sewers imposing a heavy concentrated load, a pile foundation should be constructed. The foundation may consist of piles alone, pile bents, or a wooden platform supported on pile bents. The load which can be carried by a pile is determined with accuracy only by driving a test pile and placing a load on it. Where piles do not penetrate to a firm stratum the load they will support can be determined by any one of the various formulas, either theoretical or empirical, which have been devised. Probably the best known of these formulas are the so-called Engineering News formulas one of which is:

P = 2Wh
S + 1 for a pile driven by a drop hammer,

in which P = the safe load on the pile in pounds. The factor of safety is 6; W = the weight of the hammer in pounds; h = the fall of the hammer in feet; S = the penetration of the pile in inches at the last driving blow. The blow is assumed to be driven on sound wood without rebound of the hammer.

Reference should be made to engineering handbooks for other forms of pile formulas. The accuracy of all of these formulas is not of a high degree.

The piles are driven at about 2 to 4 feet centers, to a depth of from 8 to 20 feet, unless hard bottom is struck at a lesser depth. The butt diameter of the piles used for the smallest sewers is about 6 to 8 inches. If bents are used, 2 or 3 piles are driven in a row across the line of the sewer and are capped with a timber. For brick, block, pipe, and some concrete sewers, a wooden platform must be constructed between the pile bents for the support of the sewer.

67. Underdrains.—The construction of special foundations can sometimes be avoided by laying drains under the sewers, thus removing the water held in the soil. The laying of the underdrains facilitates the construction of the sewer and reduces the amount of ground water entering the sewer. The underdrains usually consist of 6– or 8–inch vitrified tile laid with open joints from 1 to 2 feet below the bottom of the sewer as shown in Fig. 1. If the sewers are large, parallel lines of drains may be laid beneath them. An observation hole should be constructed from the bottom of the manhole to each underdrain. This hole usually consists of a 6– or 8–inch pipe, embedded in concrete, connected to the drain and open at the top. It is too small to permit effective cleaning of the underdrains, which are usually neglected after construction, and which as a result clog and cease to function. Since the principal period of usefulness of the drains is during construction, their stoppage after the work is completed is not serious. The hollow tile used in vitrified block sewers serve as underdrains after construction, but are of little or no assistance to the draining of the trench during construction.

CHAPTER VII
PUMPS AND PUMPING STATIONS

68. Need.—In the design of a sewerage system it is occasionally necessary to concentrate the sewage of a low-lying district at some convenient point from which it must be lifted by pumps. In the construction of sewers in flat topography the grade required to cause proper velocity of sewage flow necessitates deep excavation. It is sometimes less expensive to raise the sewage by pumping than to continue the construction of the sewers with deep excavation.

In the operation of a sewage-treatment plant a certain amount of head is necessary. If the sewage is delivered to the plant at a depth too great to make possible the utilization of gravity for the required head, pumps must be installed to lift the sewage. In the construction of large office buildings, business blocks, etc., the sub-basements are frequently constructed below the sewer level. The sewage and other drainage from the low portion of the building must therefore be removed by pumping. Because pumps are often an essential part of a sewerage system, their details should be understood by the engineer who must write the specifications under which they are purchased and installed.

69. Reliability.—If the only outlet from a sewerage system is through a pumping station, the inability of the pumps to handle all of the sewage delivered to them may so back up the sewage as to flood streets and basements, endangering lives and health and destroying property. Such an occurrence should be guarded against by providing sufficient pumping capacity and machinery of the greatest reliability.

70. Equipment.—The equipment of a sewage pumping station, in addition to pumping machinery, may include a grit chamber, a screen, and a receiving well. The grit chamber and screen are necessary to protect the pumps from wear and clogging. Grit chambers are not necessary in sewage devoid of gritty matter, such as the average domestic sewage, unless reciprocating pumps are used. Sufficient gritty matter is found in average domestic sewage to have an undesirable effect on reciprocating pumps. Receiving wells are used in small pumping stations where the capacity of the pumps is greater than the average rate of sewage flow. The pumps are then operated intermittently, the pumps standing idle during the time that the receiving well is filling.

Except for a few types of pumps of which the valve openings are unsuitable, any machine capable of pumping water is capable of pumping sewage which has been properly screened. The principles of sewage pumps are then similar to principles of water pumps. The conditions under which these principles are applied differ but slightly in the character of the liquid, and a smaller range of discharge pressures. Pumps with large passages, discharging under low heads are more commonly found among sewage pumps.

Fig. 49.—Calumet Sewage Pumping Station, Chicago, Illinois.

71. The Building.—The pumping station should, if possible, be of pleasing design and should be surrounded by attractive grounds. The Calumet Sewage Pumping Station in Chicago is shown in Fig. 49. Its architecture is pleasing particularly in contrast with its location and immediate surroundings. Such structures tend to remove the popular prejudice from sewerage and to arouse interest in sewerage questions. Service to the public is of value. It can be rendered more easily by arousing public interest and cooperation by the erection of attractive structures, than by feeding popular prejudice by the construction of miserable eyesores.

72. Capacity of Pumps.—The capacity of the pumping equipment should be sufficient to care for the maximum quantity of sewage delivered to it, with the largest pumping unit shut down, and the provision of such additional capacity as, in the opinion of the designer, will provide the necessary factor of safety.

Pumps can usually be operated under more or less overload. Power pumps and centrifugal pumps driven by constant speed electric motors have no overload capacity. A power pump or a centrifugal pump may be overloaded up to the maximum horse-power of any variable speed motor or steam engine driving it, provided the pump has been designed to permit it. Direct-acting steam pumps which are designed for proper piston speed and valve action at normal loads, can carry a 50 per cent overload for short periods, although the strain on the pump is great. They will carry a 20 to 25 per cent overload for about eight hours with less vibration and strain. The use of pumps capable of working at an appreciable overload is somewhat of an additional factor of safety, but the overload factor should not be taken into consideration in determining the capacity of the pumping equipment.

The load on a pumping station consists of the quantity of sewage to be pumped and the height it must be lifted. Variations in the quantity are discussed in Chapter III. The head against which the pumps must operate fluctuates with the level in the tributary sewer or pump well, and in the discharge conduit. For a free discharge or discharge into a short force main the greater the rate of sewage flow the smaller the lift, as the depth of flow in the tributary sewer increases more rapidly than that in the discharge conduit. If the discharge is into a large body of water or under other conditions where the discharge head is approximately constant, the fluctuations in total head should not exceed the diameter of the tributary sewer. Such fluctuations are of minor importance in the operation of direct-acting steam pumps, but may be of great importance in the operation of centrifugal pumps, as is brought out in Art. 78.

73. Capacity of Receiving Well.—The use of receiving wells is restricted to small installations which require, in addition to the standby unit, only one pump, the capacity of which is equal to the maximum rate of sewage flow. When the receiving well has been pumped dry the pump stops, allowing the well to fill again. Although the use of a large receiving well, or an equalizing reservoir, for a large pumping station would permit the operation of the pumps under more economical conditions, the storage of sewage for the length of time required would not be feasible. The sewage would probably become septic, creating odors and corroding the pumps. The extra cost of the reservoir might not compensate for the saving in the capacity and operation of the pumps.

The capacity of the receiving well should be so designed that the pump when operating will be working at its maximum capacity, and the period of rest during conditions of average rate of flow should be in the neighborhood of 15 to 20 minutes. For example, assume an average rate of flow of 2 cubic feet per second, with a maximum rate of double this amount. The pump should have a capacity of 4 cubic feet per second, and if the receiving well is to be filled in 15 minutes by the average rate of sewage flow its capacity should be 15 × 5 × 60 × 7.5 or 14,000 gallons. Under these circumstances the pump will operate 15 minutes and rest 15 minutes, during average conditions of flow.

74. Types of Pumping Machinery.—The two principal types of pumping machines for lifting sewage are centrifugal pumps and reciprocating pumps. A centrifugal pump is, in general, any pump which raises a liquid by the centrifugal force created by a wheel, called the impeller, revolving in a tight casing, as shown in Fig. 50. A reciprocating pump is one in which there is a periodic reversal of motion of the parts of the pump.

Centrifugal pumps are sometimes classified as volute pumps and turbine pumps. A volute pump is a centrifugal pump with a spiral casing into which the water is discharged from the impeller with the same velocity at all points around the circumference, as shown in Fig. 51. A turbine pump is a centrifugal pump in which the water is discharged from the impeller through guide passages into a collecting chamber, in such a manner as to prevent loss of energy in changing from kinetic head to pressure head. A turbine pump is shown in section in Fig. 51. Centrifugal pumps are sometimes classified as single stage and multi-stage. A centrifugal pump from which the water is discharged at the pressure created by a single impeller is called a single-stage pump. If the water in the pump is discharged from one impeller into the suction of another impeller the pump is known as a multi-stage pump. The number of impellers operating at different pressures determines the number of stages of the pump. A three-stage pump is shown in Fig. 52.

Fig. 50.—Section through de Laval Single-Stage, Double Suction Centrifugal Pump.

375 Lubricating ring. 380 Oil hole cap. 383 Oil drain tube. 404 Shaft sleeve lock nut. 440 Driving coupling. 441 Driven coupling. 443 Coupling check nut. 450 Coupling bolt. 451 Coupling bolt nut. 452 Coupling rubber. 453 Coupling rubber steel tube. 500 Pump case. 550 Bearing bracket cap. 551 Bearing. 552 Shaft. 553 Shaft sleeve, right hand thread. PW Impeller. 554 Shaft sleeve, left hand thread. 555 Shaft stop collar, inner. 555–1 Shaft stop collar, outer. 556 Guide ring. 560 Packing gland. 563 Bearing. 567R Impeller protecting ring, right hand thread. 567L Impeller protecting ring, left hand thread. 583 Pump case protecting ring. 567 Labyrinth packing. 583 Labyrinth packing. 600 Pump case cover. 692 Impeller key. 815 Bearing bracket, outer. 815–1 Bearing bracket, inner.

Fig. 51.—Types of Centrifugal Pumps.

Fig. 52.—Section of a Multi-Stage Centrifugal Pump.
Courtesy DeLaval Steam Turbine Co.

Reciprocating pumps are generally driven by steam and are either direct-acting, or of the crank-and-fly-wheel type. Power pumps are reciprocating machines which may be driven by any form of motor, to which they are connected by belt, chain or shaft. A Deming triplex power pump is shown in Fig. 53. Power pumps can be used only where the character of the sewage will not clog the valves nor corrode the pump. A direct-acting steam pump is one in which the steam and water cylinders are in the same straight line and the steam is used at full boiler pressure throughout the full length of the stroke. The crank-and-fly-wheel type of pumping engine permits the use of steam expansively during a part of the stroke, the energy stored in the flywheel carrying the machine through the remainder of the stroke. Reciprocating pumps are sometimes classified as plunger pumps and piston pumps. In the action of a plunger pump the water is expelled from the water cylinder, by the action of a plunger which only partly fills the water cylinder, as shown in Figs. 54 and 55. In a piston pump the water is expelled from the water cylinder by the action of a piston which completely fills the water cylinder, as shown in Fig. 63, which illustrates a direct-acting piston pump.

Fig. 53.—Triplex Power Pump.
Courtesy, The Deming Co.

Plungers are better than pistons for pumping sewage as the wear between the pistons and the inside face of the cylinder soon reduces the efficiency of the pump. Outside packed plungers are better than the inside packed type because the packing can be taken up without stopping the pump and the leakage from the pump is visible at all times. Outside packed pumps are more expensive in first cost, but are easier to maintain and have a longer life than piston pumps.

Fig. 54.—Water End of Inside Center-Packed Plunger Pump.

In selecting a pump to perform certain work the size of the water cylinder and the speed of the travel of the piston should be investigated to insure proper capacity. The average linear travel of the piston for slow speed pumps is estimated at about 100 feet per minute, dependent on the length of stroke and the valve area. For short strokes and small valve areas the speed may be as low as 40 feet per minute, and for long stroke fire pumps with large valves the piston can be operated at a speed of 200 feet per minute.[^[45]] Vertical, triple-expansion, crank-and-fly-wheel, outside packed plunger pumps with flap valves can be operated at speeds of 200 feet per minute when lifting sewage, and when equipped with mechanically operated valves and lifting water they can be run at speeds of 400 to 500 feet per minute. The speed of travel multiplied by the volume of piston or plunger displacement, with proper allowance for slippage, will give the capacity of the pump. The slippage allowance may be from 3 to 8 per cent for the best pumps, and for pumps in poor conditions it may be a high as 30 to 40 per cent.

Fig. 55—Water End of Outside Center-Packed Plunger Pump.
Courtesy Allis-Chalmers Co.

There are two types of ejector pumps used for lifting sewage. One of these depends on the vacuum created by the velocity of a stream of water or steam passing through a small nozzle. The operation of this pump is described in Art. 139 and it is illustrated in Fig. 97. The other type of ejector pump is known as the compressed-air ejector. It is operated by means of compressed air which is turned into a receptacle containing sewage. The details of this type are explained in Art. 83 and are illustrated in Fig. 68.

75. Sizes and Description of Pumps.—The size of a centrifugal pump is expressed as the diameter of the discharge pipe in inches. It has nothing to do with the head for which the pump is suited. On the assumption of a velocity of flow of 10 feet per second through the discharge pipe the capacity of the pump can be approximated.

The size of a reciprocating pump involves the expression of the diameters of the steam cylinders, the water cylinder, and the length of the stroke in inches, in the order named, beginning with the steam cylinder with the highest pressure. A complete description of a steam pumping engine might be; compound, duplex, horizontal, condensing, crank-and-fly-wheel, outside-center-packed, 12″ × 24″ × 18″ × 24″ pump. The word compound means that there are a high-pressure and a low-pressure steam cylinder; the word duplex means that there are two of each of these cylinders; the word horizontal means that the axes of these cylinders are in a horizontal plane; the word condensing means that the steam is discharged from the low-pressure cylinders into a condenser; the name crank-and-fly-wheel is self-explanatory; the name outside-center-packed means that the water cylinder is divided into two portions between which the plunger is exposed to the atmosphere, and that the packing rings are on the outside of the two portions of the cylinder as shown in Fig. 55; the figures shown mean that the high-pressure steam cylinder is 12 inches in diameter, the low-pressure 24 inches in diameter, the water cylinder is 18 inches in diameter, and the stroke of the pump is 24 inches.

76. Definitions of Duty and Efficiency.—The duty of a pump is the number of foot-pounds of work done by the pump per million B.T.U., per thousand pounds of steam, or per hundred pounds of coal, consumed in performing the work. These units are only approximately equal as 100 pounds of coal or 1,000 pounds of steam do not always contain the same number of B.T.U. and may only approximately equal 1,000,000 B.T.U.

Since 1,000,000 B.T.U. are equal to 778,000,000 foot-pounds of work, a pump with a duty of 778,000,000 will have an efficiency of 100 per cent. The efficiency of a pump is therefore its duty based on B.T.U. divided by 778,000,000. The efficiencies or duties of various types of pumps are given in Table 26.[^[46]]

TABLE 26
Approximate Duties of Steam Pumps
Small duplex, non-condensing	10,000,000
Large duplex, non-condensing	25,000,000
Small simple, flywheel, condensing	50,000,000
Large simple, flywheel, condensing	65,000,000
Small compound, flywheel, condensing	65,000,000
Large compound, flywheel, condensing	120,000,000
Small triple, flywheel, condensing	150,000,000
Large triple, flywheel, condensing	165,000,000

77. Details of Centrifugal Pumps.—A section of a centrifugal pump with the names of the parts marked thereon is shown in Fig. 50. Among the important parts which require the attention of the purchaser are: the impeller (PW), the impeller packing rings (567 R & L), the bearings (551, 563), the thrust bearings (555–1), the shaft (552), and the shaft coupling (440).

The impeller should be of bronze, gun metal, or other alloy, because there is no rusting or roughening of the surface, and the efficiency does not fall with age. Normal fresh sewage is not corrosive, but septic sewage and sludge are usually so corrosive that iron parts cannot be used with success in contact with them. The impeller should be machined and polished to reduce the friction with the liquid. Impellers are made either closed or open, i.e., either with or without plates on the sides connecting the blades to avoid the friction of the liquid against the side of the casing. The closed type of impeller is shown in Fig. 50. Closed impellers are slightly more expensive, but generally give better service and higher efficiencies than the open type. Single impeller pumps should have an inlet on each side of the impeller to aid in balancing the machine, unless the plane of the impeller is to be horizontal when operating. Multi-impeller pumps usually have single inlet openings for each impeller. Vibration in the pump is sometimes caused by an unbalanced impeller. The moving parts may be balanced at one speed and unbalanced at another. To determine if the moving parts are balanced the pump should be run free at different speeds and the amount of vibration observed. If the impeller is removed from the pump its balance when at rest can be studied by resting it on horizontal knife edges. If there is a tendency to rotate in any direction from any position the impeller is not perfectly balanced.

Packing rings are used to prevent the escape of water from the discharge chamber back into the suction chamber. These rings should be made of the same material as the impeller. Labyrinth type rings, as shown in Fig. 50, are sometimes used as the long tortuous passages are efficient in preventing leakage.

The bearings must be carefully made because of the high speed of the pump. They are usually made of cast iron with babbitt lining. They should be placed on the ends of the shaft on the outside of the pump casing, as shown in Fig. 50, and should be split horizontally so as to be easily renewed. Exterior bearings are oil lubricated by means of ring or chain oilers with deep oil wells. Where interior bearings are necessary, because of the length of the shaft, they should be made of hard brass and should be water lubricated.

Fig. 56.—Marine Type Thrust Bearing.
Courtesy, DeLaval Steam Turbine Co.

Thrust bearings or thrust balancing devices are used to take up the end thrust which occurs in even the best designed pumps. To overcome this pumps are designed with double suction, opposed impellers, or two pumps with their impellers opposed may be placed on the same shaft. Due to inequalities in wear, workmanship or other conditions, end thrust will occur and must be cared for. Various types of thrust bearings are in successful use, such as: the piston, ball, roller or marine types. The marine type thrust bearing is shown in Fig. 56. The piston type of hydraulic balancing device is shown in Fig. 57. In the figure A represents the impeller, and B a piston fixed to the shaft and revolving with it. There is a passage for water through the openings (1), (2), and (3) leading from the impeller chamber to the atmosphere or to the suction of the pump. If the impeller tends to move to the right opening (1) is closed resulting in pressure on the right of the impeller forcing it to the left. If the impeller moves to the left (1) is opened thus transmitting pressure to the piston B forcing the impeller to the right. The flange C is not essential, but is advantageous in pumps handling gritty matter. As the channel (2) wears larger the pressure against the piston decreases allowing it to move to the left. This partially closes (3) building up the pressure again.

Fig. 57.—Piston Type of Thrust Balancing Device.

Flexible shaft couplings should be used if the shaft of the driving motor and the pump are in the same line, as direct alignment is difficult to obtain or to maintain. Where connected to steam turbines, reduction gearing and rigid couplings are usually used on sewage pumps to obtain slow speed and permit large passages. Flexible couplings are of various types, one of which is shown in Fig. 50. A rigid coupling would be formed by bolting the flanges firmly together. Shaft couplings are usually not necessary where the pump is driven by belt connection to the engine or motor, or where the pump and pulley rest on only two bearings.

The stuffing box shown in Fig. 50 is packed loosely with two layers of hemp between which is a lantern gland, in order to permit a small amount of leakage. A drip box is placed below this gland to catch the leakage and return it to the pump. The leakage is permitted as it aids in lubrication and the tightening of the gland will cause binding of the shaft. The gland on the suction side of the pump should be connected by a small pipe to the discharge chamber in order to keep a constant supply of water for lubrication and to prevent the entrance of air to the suction end of the pump.

78. Centrifugal Pump Characteristics.—The capacity of a centrifugal pump is fixed by the size and type of its impeller and by the speed of revolution. Roughly, the capacity of a pump, for maximum efficiency, varies directly as the speed of revolution, the discharge pressure varies as the square of the speed, and the power varies as the cube of the speed. These relations are found not to hold exactly in tests because of internal hydraulic friction in the pump.

The characteristic curves for a centrifugal pump, or the so-called pump characteristics, are represented graphically by the relation between quantity and efficiency, quantity and power necessary to drive, and quantity and head, all at the same speed. The quantities are plotted as abscissas in every case. The curve whose ordinates are head and whose abscissas are quantities is known as “the characteristic.” The curve showing the relation between quantities and speeds is sometimes included among the characteristics. Characteristics of pumps with different styles of impellers are shown in Fig. 58. Fig. 59 shows the characteristics of a pump run at different speeds, the efficiencies at these speeds when pumping at different rates, and the maximum efficiency at different speeds. It is to be noted that the information given in this figure is more extensive than that in Fig. 58. The operating conditions under any head, rate of discharge, and speed are given. The curves of constant speed are parallel, and their distances apart vary as the square of the speed. The line of maximum efficiency is approximately a parabola.

Fig. 58.—Characteristics of Centrifugal Pumps with Different Styles of Impellers at Constant Speed.

A study of the characteristics of any particular pump should be made with a view to its selection for the load and conditions under which it is to be used. Among the important things to be considered in the selection of a centrifugal pump for the expected conditions of load are: the capacity required, the maximum and minimum total head to be pumped against, the maximum variations in suction and discharge heads, and the nature of the drive. For example, the pump, whose characteristics are shown in Fig. 59, should be operated at about 800 revolutions per minute. Under total heads between 40 and 50 feet, the discharge for the best efficiency will vary between 600 and 670 gallons per minute.

Fig. 59.—Efficiency and Characteristic Curves of a Centrifugal Pump at Different Speeds.

Fig. 60.—Efficiencies of Centrifugal Pumps.

The efficiencies of centrifugal pumps increase with their capacities as is shown approximately in Fig. 60.

79. Setting of Centrifugal Pumps.—In setting a centrifugal pump, care should be taken to provide a firm foundation to hold the shafts of the pump and the electric motor or the reduction gearing in good alignment, or to prevent the pump from being displaced by the pull of a belt. It is desirable that the foundation be level. Centrifugal pumps should be set submerged for small pumping stations automatically controlled. Sludge pumps must be set submerged as otherwise they will not prime successfully. Provision should be made by which the pump can be lifted from the sewage, or sludge, for inspection and repair. In many cases the pump can be made self-priming by setting it in a dry, water-tight vault below the low level of sewage flow. Where possible it is desirable not to set the pump submerged as it will receive better care when easily accessible.

Fig. 61.—Centrifugal Pump in Manhole at Duluth, Minn.
Eng. Contracting, Vol. 43, 1915, p. 310.

The suction pipe should be free from vertical bends where air might collect and should be straight for at least 18 to 24 inches from the pump casing. An elbow on the suction pipe, attached directly to the casing of the pump gives a lower efficiency than a suction pipe with a short straight run. Centrifugal pumps will operate with as high a suction lift as reciprocating pumps, but at the start they must be primed and some provision must be made for priming them. The suction pipe should be equipped with foot valves to hold the priming, or some method may be provided for exhausting the air from the suction pipe. The foot valves should be so installed as to form no appreciable obstruction to the flow of water. They should have an area of opening at least 50 per cent greater than the cross-section of the suction pipe. A strainer on the suction pipe is undesirable as it becomes clogged and is usually in an inaccessible position for cleaning. A screen should be placed at the entrance to the suction well to prevent the entrance of objects that are likely to clog the pump. A gate-valve and a check-valve should be provided on the discharge pipe, the former to assist in controlling the rate of discharge and the latter to prevent back flow into the pump when it is not operating.

Centrifugal pumps are well adapted to service in either large or small units. An installation in a manhole at Park Point, Duluth, is shown in Fig. 61. This station is controlled by an automatic electric device which is operated by a float in the suction pit. Such automatic control is an added advantage of the use of electrically driven centrifugal pumps. The Calumet Pumping Station in Chicago, shown in Fig. 49, has a capacity of approximately 1,000 cubic feet per second. The simplicity of the layout of this station is shown in Fig. 62.

Fig. 62.—Interior Arrangement of the Calumet Sewage Pumping Station, Chicago.
Eng. News-Record, Vol. 85, 1920, p. 872.

80. Steam Pumps and Pumping Engines.—The direct-acting steam pump, one type of which is shown in Fig. 63, is adapted to pumping sewage the character of which will not corrode or clog the valves. In this form of pump it is necessary to utilize the steam at full pressure throughout the entire length of the stroke, which results in high steam consumption. A flywheel permits the use of steam expansively during a part of the stroke, thus increasing the economy of operation. Other devices used for the same purpose are known as compensators. They are not in general use.

Steam engines are classified in many different ways, for example; according to the type of valve gear, as, plain slide valve, Corliss, Lentz, etc.; or according to the number of steam expansions, as, simple, compound, triple-expansion, etc.; or according to the efficiency of the machine as low duty or high duty; or as

Fig. 63.—Section of Duplex Piston Steam Pump.
Courtesy, The John H. McGowan Co.

STEAM END

2 Steam cylinder and housing combined. 8 Steam piston head. 9 Steam piston follower. 10 Steam piston inside ring. 11 Steam piston outside ring (2). 12 Steam cylinder head. 14 Steam chest. 16 Steam chest cover. 17 Steam slide valve. 18 Steam valve rod. 20 Steam valve rod, pin and nut. 22 Steam valve rod, collar and set screw. 23 Steam valve rod, stuffing box. 24 Steam valve rod, stuffing box, nut and gland. 38 Piston rod. 47 Piston rod stuffing box. 48 Piston rod, stuffing box, nut and gland. 49 Valve gear stand. 51 Long valve crank and shaft. 52 Short valve crank and shaft.

PUMP END

115 Pump body. 127 Brass liner. 129 Water piston head. 130 Water piston follower. 137 Cylinder head. 139 Valve plate. 140 Cap. 152 Suction flange. 161 Discharge flange. 162 Valve seat, suction or discharge. 163 Valve, suction or discharge. 164 Suction valve spring. 167 Discharge valve spring. 168 Valve plate, suction or discharge. 169 Valve stem, suction or discharge.

STEAM END

55 Crank pin. 56 Valve rod link. 61 Long rocker arm. 62 Short rocker arm. 63 Rocker arm wiper. 69 Cross head.

condensing or non-condensing, etc. Throttling engines or automatic engines refer to the method of control of the steam by the governor. In throttling engines the governor controls the amount of opening of the throttle valve, in automatic engines the governor controls the position of the cut-off.

The simple slide valve, low-duty, non-condensing, throttling engine, is the lowest in first cost and the most expensive in the consumption of fuel. The triple-expansion Corliss, or the non-releasing Corliss, high-duty pumping engine is the most expensive in first cost but consumes less steam for the power delivered than any other form of reciprocating engine. For pumps of very small capacity the cost of fuel is not so important an item as the first cost of the machine. For this reason and because of the lower cost of attendance low-duty pumps are more frequently found in small pumping stations.

Fig. 64.—Diagram Showing Rates of Steam Consumption for Different Size Units under Different Loads.

TABLE 27
Water Rates of Prime Movers at Full and Part Loads
Type of Engine	Power, K.W.	Per Cent of Full Load					Boiler Press. Lbs.
		25	50	75	100	125
Single cylinder, high speed, non-condensing	25	33	27	26.3	27.0	27.5	100 to 150
	250	42	37.5	35	34.0	34.0
Automatic, flat four valve, high speed	150		32	30	26.5	29.0	100 to 125
	250		33	31	28	30.0
Tandem compound condensing, high speed	125		23	19	17	18	100 to 150
			25	20	19.5	21
Cross compound, condensing, high speed		30	26	24	23	23.5	125
Cross compound, non-condensing, high speed		39	31	27	26	27.5	125
Single cylinder Corliss, condensing	120	23.7	20.4	19	18.5	19.0	100
	500	26.3	22.8	21.3	20.8	21.3	125
Compound Corliss, condensing		16.5	14	12.5	12.1	12.5	100
		22.2	19	17.0	16.5	17.0	150
Single cylinder, rotary four valve, non-condensing	75	26.2	22.3	21.3	21.6	22.8	100
	400	35.0	27.2	26.4	26.0	26.8	180
Rotary four valve, tandem compound non-condensing	125	32.0	22.0	20	18.25	18.5	100
	600	40.0	28.3	23.2	22.5	22.7	150
Cross compound, non-condensing rotary four valve	125	25	21	19.1	18.5	19.0	100
	600	39.4	28	22.3	20.6	20.7	150
Single cylinder, poppett valve, non-condensing	120	22.7	20.5	19.7	19.1	20.1	100
	600	28.5	26.0	25.0	24.3	25.5	150
Single cylinder, poppett valve, condensing	120	18.5	16.7	16.1	15.6	16.4	100
	600	24.6	22.3	21.4	20.8	21.9	150
Compound condensing, poppett valve	200	14.2	13.0	12.5	12.2	12.9	100
	1200	18.4	16.9	16.3	15.9	16.8	150
Uniflow	125	14.6	13.7	13.4	13.4	13.3	150
	600	15.0	14.3	13.7	13.5	14.0
Steam turbines, condensing, Allis-Chalmers	300		24	17	160	16.5	125
	2000		31.9	26.3	23.8	23.0	175
Steam turbines, condensing, Westinghouse	300		13.7	12.8	12.2	12.6	125
	2000		18.2	16.9	16.2	16.8	175
Steam turbines, high pressure, non-con., 12″ to 36″ wheel, 1000 to 3600 R.P.M.	4 to 8 stages				28 5
					116.5
Ditto. Condensing, 26–inch					17 3
					112.0

The steam consumption per indicated horse-power, better known as the water rate of the engine, for various types of engines at full and at part load is shown in Fig. 64. The steam consumption of other types at full load is shown in Table 27. The indicated horse-power (I.H.P.) of a steam engine is the product of the mean effective pressure (M.E.P.), the area of the steam pistons, the length of the stroke, and the number of strokes per unit of time. A common form of this expression is,

I.H.P = PLAN
33,000,

in which P = the M.E.P. in pounds per square inch; L = the length of the stroke in inches; A = the sum of the areas of the pistons in square inches; N = the number of revolutions per minute.

The I.H.P. multiplied by the mechanical efficiency of the machine will give the brake or water horse-power, that is, the horse-power delivered by the machine. The product of the M.E.P., the sum of the areas of the steam pistons and the mechanical efficiency of the machine, should equal the product of the total head of water pumped against expressed in pounds per square inch and the sum of the areas of the water pistons or plungers. The M.E.P. is determined from indicator cards taken from the steam cylinders during operation. These cards show the steam pressure on the head and crank ends of each cylinder at all points during the stroke.

81. Steam Turbines.—Among the advantages in the use of steam turbines as compared with reciprocating steam engines for driving centrifugal pumps are their simplicity of operation, the small floor space needed, their freedom from vibration requiring a relatively light foundation, and their ability to operate successfully and economically either condensing or non-condensing under varying steam pressure. They can be operated with steam at atmospheric or low pressure, thus taking the exhaust from other engines. The greatest economy of operation for the turbine alone will be obtained by operating with high pressure, superheated steam and with a vacuum of 28 inches. In large units the economy of operation of steam turbines is equal to that of the best type of reciprocating engines. In order to develop the highest economy turbines are operated at speeds from about 3,600 to 10,000 r.p.m. or greater, the smaller turbines operating at the higher speeds. As these speeds are usually too great for the operation of centrifugal pumps for lifting sewage, reduction gears must be introduced between the turbine and the pump. Although the best form of spiral-cut reduction gears may obtain efficiencies of 95 to 98 per cent, or even higher, their use, particularly in small units, is an undesirable feature of the steam turbine for driving pumps.

The steam consumption of DeLaval turbines of different powers, and the steam consumption of a 450 horse-power DeLaval turbine at different loads are shown in Fig. 64. Some steam consumptions of other turbines are recorded in Table 27. It is to be noted that the steam consumption of the 450 horse-power turbine at part loads is not markedly greater than that at full loads. This is an advantage of steam turbines as compared with reciprocating engines. The steam consumption of any turbine is dependent on the conditions of operation and is lower the higher the vacuum into which the exhaust takes place.

Fig. 65.—The DeLaval Trade Mark, Illustrating the Principle of the DeLaval Steam Turbine.
Courtesy, DeLaval Steam Turbine Co.

There are two types of turbines in general use, the single stage or impulse machines, and the compound or reaction type. The DeLaval is a well-known make of the single stage or impulse type. The principle of its operation is indicated in Fig. 65, which is the trade mark of the DeLaval Steam Turbine Co. The energy of the steam is transmitted to the wheel due to the high velocity of the steam impinging against the vanes. In the compound or reaction type of machine the steam expands from one stage to the next imparting its energy to the wheel by virtue of its expansion in the passages of the turbine. For this reason the single-stage or impulse type is operated at higher speeds than the compound or reaction machines.

82. Steam Boilers.—Among the important points to be considered in the selection of a steam boiler for a sewage pumping station are: the necessary power; the quality of the feed water; the available floor space; the steam pressure to be carried; and the quality and character of the fuel. Tubular boilers of the type shown in Fig. 66, are lower in first cost than other types of boilers. They are not ordinarily built in units larger than 250 to 300 horse-power and where more power is desired a number of units must be used. They are objectionable because of the relatively large floor space required, and because of their relatively poor economy of operation. The efficiencies of water-tube boilers of different types are given in Table 28. Large power units of the water-tube type, as shown in Fig. 67, although more expensive in first cost, require less floor space. Almost any desired steam pressure can be obtained from either type but water-tube boilers are more commonly used for high pressures. The grate or stoker can be arranged to burn almost any kind of fuel under either water-tube or fire-tube boilers. The use of poor quality of water in water-tube boilers is undesirable as the tubes are more likely to become clogged than the larger passages of the fire-tube boilers. If necessary, a feed-water purification plant should be installed, as it is usually cheaper to take the impurities out of the water than to take the scale out of the boiler.

Fig. 66.—Horizontal Fire-tube Boiler.

Fig. 67.—Babcock and Wilcox Water-tube Boiler.

Not less than two boiler units should be used in any power station, regardless of the demands for power, and if the feed water is bad, three or even four units should be provided, as two units may be down at any time. An appreciable factor of safety is provided by the ability of a boiler to be operated at 30 to 50 per cent overload, if sufficient draft is available, but with resulting reduction in the economy of operation. The number of units provided should be such that the maximum load on the pumping station can be carried with at least one in every 6 units or less, out of service for repairs or other cause.

TABLE 28
Efficiencies of Steam Boilers
From Marks’ Mechanical Engineer’s Handbook
Type	Horse-power	Furnace	Sq. Ft. Grate Area	Per Cent of Rated Capacity D’v’l’d	B.T.U. per Lb. Dry Coal	Evap. from and at 212° per Lb. Dry Coal	Combined Efficiency of Boiler and Furnace
Babcock & Wilcox	300	Hand-fired	84	118.7	11,912	8.81	71.8
Babcock & Wilcox	640	Hand-fired	118	121.5	14,602	10.83	72.0
Stirling	1128	B. & W. chain grate	187	198.3	12,130	9.51	76.1
Rust	335	Hand-fired	68	210.5	13,202	9.42	68.9
Heine	400	Green chain grate	83.5	123.8	11,608	8.79	73.5
Maximum efficiency recorded							83

The steam delivered by a boiler is the basis of the measurement of its capacity or power. A boiler horse-power is the delivery of 33,320 B.T.U. per hour. It is approximately equal to the raising of 30 pounds of water per hour from a temperature of 100° Fahrenheit, to steam at a pressure of 70 pounds per square inch, or to 34 pounds of water per hour changed to steam from and at 212° Fahrenheit, at atmospheric pressure. The horse-power of a boiler is sometimes approximated by the area of its grate or heating surface. Such a method of measuring has a low degree of accuracy on account of the variations in the quality of the fuel, and the rate of combustion. For example, the rate of combustion under a locomotive boiler is high and there is less than ⅒th of a square foot of grate area and about 4.5 square feet of heating surface per boiler horse-power. The Scotch Marine type of boiler used on steam ships, has slightly more grate area and slightly less heating surface than the locomotive type of boiler, because the rate of combustion is lower. Stationary water-tube boilers may have 2 to 3 times as much grate area and heating surface per horse-power as is found in locomotive boilers. If a poor type of fuel is to be used the area of the grate should be increased about inversely as the heat content of the fuel. The approximate heat content of various types of fuels is shown in Table 29.

TABLE 29
Approximate Heat Value of Fuels
Fuel	B.T.U. per Pound	Pounds of Water Evaporated from and at 212° F. All heat utilized
Anthracite	13,500	14.0
Semi-bituminous, Pennsylvania	15,000	15.5
Semi-bituminous, best, West Virginia	15,000	15.8
Bituminous, best, Pennsylvania	14,450	15.0
Bituminous, poor, Illinois	10,500	10.9
Lignite, best, Utah	11,000	11.4
Lignite, poor, Oregon	8,500	8.8
Wood, best oak	9,300	9.6
Wood, poor ash	8,500	8.8

83. Air Ejectors.—The Ansonia compressed-air sewage ejector is shown in Fig. 68. In its operation, sewage enters the reservoir through the inlet pipe at the right, the air displaced being expelled slowly through the air valve marked B. The rising sewage lifts the float which actuates the balanced piston valve in the pipe above the reservoir when the reservoir fills. The lifting of the valve admits compressed air to the reservoir. The air pressure closes valve A and the inlet valve at the right, and ejects the sewage through the discharge pipe at the left. As the float drops with the descending sewage it shuts off the air supply and opens the air exhaust through the small pipe at the top center. Sewage is prevented from flowing back into the reservoir by the check valve in the discharge pipe. Other ejectors operating on a similar principle are the Ellis, the Pacific, the Priestmann and the Shone.

84. Electric Motors.—The most common form of alternating current electric motor used for driving sewage pumps where continuous operation and steady loads are met is the squirrel-cage polyphase induction motor. These motors operate at a nearly constant speed which should be selected to develop the maximum efficiency of the pump and motor set. While Fig. 59 shows the best efficiency under varying heads to be obtained with variable speed, the advantages of cost, attention, and availability make the use of a constant speed motor common.[^[47]] This type of motor is undesirable where stopping and starting are frequent because it has a relatively small starting torque and it requires a large starting current. Such motors can be constructed in small sizes for high starting torques by increasing the resistance of the rotor, but at the expense of the efficiency of operation.

Fig. 68.—Ansonia Compressed-Air Sewage Ejector.

Alternating current motors are more generally used than direct-current motors because of the greater economy of transmission of alternating current, but where direct current is available constant speed shunt wound motors should be adopted.

In the selection of a motor to drive a centrifugal pump it is important that the motor have not only the requisite power, but that its speed will develop the maximum efficiency from the pump and motor combined. If the pump and motor operate on the same shaft the speed of the two machines must be the same. If the two are belt connected, the size of the pulleys may be selected so as to give the required speed. If the motor is to be connected to a power pump an adequate automatic pressure relief valve should be provided on the discharge pipe from the pump, to prevent the overloading of the motor or bursting of the pump in case of a sudden stoppage in the pipe. The motor must be selected to suit the conditions of voltage, cycle, and phase on the line. Transformers are available to step the voltage up or down to practically any value. Rotary converters are used to change direct to alternating current or vice versa.

85. Internal Combustion Engines.—Internal combustion engines are used for driving pumps. Units are available in size from fractions of 1 horse-power to 2,000 horse-power or more, although the use of the larger sizes is exceptional. These engines are not commonly used for sewage pumping but when used they are ordinarily belt connected to a centrifugal pump, or to an electric generator which in turn drives electric motors which operate centrifugal pumps. This type of engine is more commonly adapted to small loads, although not entirely confined to this field, as they serve admirably as emergency units to supplement an electrically equipped pumping station. The fuel efficiency of internal combustion engines is higher than for steam engines as is indicated in Table 30, but the fuel is more expensive.

The four-cycle gas engine shown in Fig. 69 is the type most commonly used. Its horse-power is the product of: the mean effective pressure, the length of the stroke, the area of the piston, and the number of explosions per second divided by 550. The M.E.P. is dependent on the character of the fuel used and the compression of the gas before ignition. Producer gas will furnish mean effective pressures between 60 and 70 pounds per square inch, natural gas and gasoline, 85 to 90 pounds per square inch, and alcohol from 95 to 110 pounds per square inch.

TABLE 30
Comparative Fuel Costs for Prime Movers
Type of Engine		Quantity of Fuel per H.P. Hour	Cost of Fuel in Cents per Horse-power Hour
Reciprocating steam engines, simple, non-condensing, 25 to 200 H.P.		21 to 8 lb. coal	4.2 to 1.6
	Triple condensing, 2000 to 10,000 H.P.	2.3 to 1.9 lb. coal	0.46 to 0.37
Steam turbines, high pressure, non-condensing,
	200 to 500 K.W.	6.5 to 4.2 lb. coal	1.3 to 0.86
	500 to 3000 K.W.	2.6 to 1.9 lb. coal	0.52 to 0.37
	Condensing 5000 to 20,000 K.W.	1.8 to 1.43 lb. coal	0.36 to 0.28
Gas engines
	Natural gas, 50 to 200 H.P.	19 to 11 cu. ft.
	Producer gas, 50 to 200 H.P.	2 to 1.5 cu. ft.
	Illuminating gas, 10 to 75 H.P.	26 to 19 cu. ft.	2.1 to 1.5
	Gasoline, 10 to 75 H.P.	1.5 to 0.8 pints	5.6 to 3.0
Oil engines, 100 to 500 H.P.		1.1 to 0.75 lb. oil
Note.—Coal assumed at $4.00 per ton, illuminating gas at 80 cents per thousand cubic feet, and gasoline at 30 cents per gallon.

Fig. 69.—Bessemer Oil Engine. Twin Cylinder, Valve Side.

The Diesel Engine is the most efficient of internal combustion engines. The original aim of the inventor, Dr. Rudolph Diesel, was to avoid the explosive effect of the ordinary internal combustion engine by injecting a fuel into air so highly compressed that its heat would ignite the fuel, causing slow combustion of the fuel thus utilizing its energy to a greater extent. The fuel and air were to be so proportioned as to require no cooling. Although the ideal condition has not been attained, the heat efficiency of Diesel engines is high. They will consume from 0.3 to 0.5 of a pound of oil (containing 18,000 B.T.U. per pound) per brake horse-power hour, giving an effective heat efficiency of 25 to 30 per cent. Although not now in extensive use in the United States it is probable that this engine will be more generally adopted for conditions suitable for internal combustion engines.

86. Selection of Pumping Machinery.—Centrifugal pumps are particularly adapted to the lifting of sewage because of their large passages, and their lack of valves. The low lifts, nearly constant head, and the possibility of equalizing the load by means of reservoirs are particularly suited to efficient operation of centrifugal pumps. They require less floor space than reciprocating pumps of the same capacity, and because of their freedom from vibration they do not demand so heavy a foundation. The discharge from the pump is continuous thus relieving the piping from vibration. In case of emergency the discharge valve can be shut off without shutting down the pump, an important point in “fool proof” operation.

Volute pumps are better adapted to pumping sewage as their passages are more free and they are better suited to the low lifts met. Gritty and solid matter will cause wear on the diffusion vanes of turbine pumps in spite of the most careful design. Although turbine pumps can possibly be built with higher efficiency than volute pumps, their efficiency at part load falls rapidly and the fluctuations of sewage flow are sufficient to affect the economy of operation. Turbine pumps are more expensive and heavier than volute pumps on account of the increased size necessitated by the diffusion vanes.

Multi-stage pumps are used for high lifts and are seldom if ever required in sewage pumping. As ordinarily manufactured, each stage is good for an additional 40 to 100 pounds pressure, but wide variations in the limiting pressures between stages are to be found.

Reciprocating plunger pumps are sometimes used for sewage pumping where the character of the sewage is such that the valves will not be clogged nor parts of the pump corroded. These pumps are seldom used in small installations or for low lifts. They are not adapted to automatic or long distance control as are electrically driven centrifugal pumps. The use of reciprocating pumps for sewage pumping is practically restricted to very large pumping stations with capacities in the neighborhood of 50,000,000 gallons per day or more. Steam-driven pumps are the most common of the reciprocating type, but power pumps are sometimes used in special cases for small installations and may be driven by either a steam or gas engine or an electric motor.

Compressed air ejectors, as described in Art. 83 are used for lifting sewage and other drainage from the basement of buildings below the sewer level.

Centrifugal pumps electrically driven are, as a rule, the most satisfactory for sewage pumping. Electric drive lends itself to control by automatic devices, which are particularly convenient in small pumping stations. The control can be arranged so that the pump is operated only at full load and high efficiency, and when not operating no power is being consumed, as is not the case with a steam pump where steam pressure must be maintained at all times. The electric driven pump is thrown into operation by a float controlled switch which is closed when the reservoir fills, and opens when the pump has emptied the reservoir. The choice between steam and electric power for large pumping stations is a matter of relative reliability and economy.

The selection of the proper type of pump, whether reciprocating or otherwise, requires some experience in the consideration of the factors involved. Fig. 70 is of some assistance. In discussing this figure, Chester states:

“Fig. 70 attempts to represent graphically, the writer’s ideas under general conditions, of the machines that should be selected for certain capacities for both principal engine and alternate and the station duty they may be expected to produce, but you must realize that this intends the principal engine doing at least 90 per cent of the work and that the head, the cost of coal, the load factor, the cost of real estate ... the boiler pressure, and the space available, and finally ... the funds available, are factors which may shift both the horizontal and curved lines. In the field of low service pumps of 10,000,000 capacity or over, the centrifugal pump reigns supreme, and for constant low heads of 20,000,000 capacity or over the turbine driven centrifugal usurps the field.”

A reciprocating pump of any type would have to be specially built for pumping sewage not carefully screened or otherwise treated, as the valves, ordinarily used in such pumps for lifting water, would clog. The vertical triple-expansion pumping engine with special valves and for large installations, and the centrifugal pump for large or small installations are the only suitable types for pumping sewage. With steam turbine or electric drive the centrifugal has the field to itself.

Fig. 70.—Expectancy Curves for Pumping Engines Working against a Pressure of 100 Pounds per Square Inch.
J. N. Chester, Journal Am. Water Works Ass’n, Vol. 3, 1916, p. 493.

87. Costs of Pumping Machinery.—The cost of pumping machinery can not be stated accurately as the many factors involved vary with the fluctuations in the prices of raw materials, transportation, labor, etc. The actual purchase price of machinery can be found accurately only from the seller. The costs given in this chapter are useful principally for comparative purposes and for exercise in the making of estimates. The costs of complete pumping stations are shown in Table 31.[^[48]] These figures represent costs in 1911.

TABLE 31
Costs of Complete Pumping Stations
These costs include the best type of triple-expansion engines, high-pressure boilers, brick or inexpensive stone building with slate roof, chimney and intake. Cost of land is not included.
Discharge Pressure, Lbs. per Sq. In.	Horse-power per Million Gals. Pumped	Cost, Dollars per Horse-power	Cost, Dollars per Million Gallons
30	12	562	6,750
40	16	438	7,000
50	20	362	7,250
60	24	312	7,500
70	28	277	7,750
80	32	250	8,000
90	36	229	8,250
100	40	213	8,500
110	44	200	8,750
120	48	187	9,000
130	52	192	10,000

88. Cost Comparisons of Different Designs.—In the design of a pumping station and its equipment the relative costs of different designs should be compared, and the least expensive design selected, due consideration being given to serviceability, reliability, and other factors without definite financial value. In comparing the costs of different types of machinery, all items in connection with the pumping station should be considered. For example, the cost of an electrically driven centrifugal pump and equipment may be less than the total cost of a steam driven reciprocating pump and equipment because of the saving in the cost of boilers, boiler house, etc., but a comparison of the capitalized cost of the two might show in favor of the reciprocating steam pump because of the lower cost of operation.

The total cost of a plant, or any portion thereof, may be considered as made up of three parts: (1) The first cost, (2) operation and maintenance and, (3) renewal. The total cost S can be expressed as

S = C + O
r + R,

in which C = the first cost; O = the annual expenditure for operation and maintenance; R = the amount set aside to cover renewal; r = the rate of interest.

S is called the capitalized cost of a plant. The annual payment necessary to perpetuate a plant is

A = Sr = Cr + O + Rr.

The value of R is useful when expressed in terms of the life of the plant or machine and the current rate of interest. It is sometimes called the depreciation factor or capitalized depreciation. If it is borne in mind that R is the amount to be set aside at compound interest for the life of the plant, at the end of which time the accrued interest should be sufficient to renew the plant, it is evident that

R(1 + R)ⁿ − R = C

or R = C
(1+r)ⁿ − 1

in which n is the period of usefulness, or life of the plant, expressed in years, no allowance being made for scrap value.

A comparison of the annual expense of three different plants is shown in Table 32. It is evident from this comparison that the machinery with the least first cost is not always the least expensive when all items are considered.

A sinking fund is a sum of money to which additions are made annually for the purpose of renewing a plant at the expiration of its period of usefulness. The annual payment into the sinking fund is equivalent to the term Rr in the expression for annual cost, or in terms of C, r, and n, the annual payment is

Cr
(1 + r)ⁿ − 1.

It is the same as the capitalized depreciation multiplied by the rate of interest. The expression r
(1 + r)ⁿ − 1 is sometimes called the rate of depreciation.

The present worth of a machine is the difference between its first cost and the present value of the sinking fund. If m represents the present age of a plant in years, then the present worth is

P = C(1 – (1 + r)ⁿ − 1
(1 + r)^m − 1).

TABLE 32
Comparison of Costs of Three Different Pumping Stations. Nominal Capacity Thirty Million Gallons per Day Raised Thirty Feet
Equipment	Plant A				Plant B				Plant C
	One Acre of Land. Brick Building, Steel Trussed Roof, Slate Covered. Cross Compound Condensing Horizontal Pumping Engine				One Acre of Land. Brick Building. Steel Trussed Roof, Slate Covered. Compound Condensing Low Duty Horizontal Pumping Engine				One Acre of Land. Frame Building, Shingle Roof. Compound Duplex Non-Condensing Pumping Engine.
	Annual Payment on First Cost	Years of Usefulness	Sinking Fund Payment	Total	Annual Payment on First Cost	Years of Usefulness	Sinking Fund Payment	Total	Annual Payment on First Cost	Years of Usefulness	Sinking Fund Payment	Total
Land	100		0	100	100		0	100	100		0	100
Permanent Structures[[49]]	1188	50	1080	2,260	1180	50	1080	2,260	810	50	775	1,585
Pumps and Machinery	440	15	435	875	390	15	395	785	360	15	352	712
Boilers	280	10	446	726	252	10	400	652	308	10	490	798
Labor				14,000				14,000				14,000
Fuel				5,500				7,200				8,200
Repairs, etc.				480				400				550
Total				23,941				25,497				25,945

Where straight-line depreciation is spoken of it is assumed that the worth of a machine depreciates an equal part of its first cost each year. For example, if the life of a plant is assumed to be 20 years, straight-line depreciation will assume that the plant loses 1
20 of its original value annually. The present worth of a plant under this assumption would be the product of its first cost and the ratio between its remaining life and its total life. This method of estimating depreciation and worth is frequently used, particularly for short-lived plants and for simplicity in bookkeeping, but it is less logical than the method given above.

89. Number and Capacity of Pumping Units.—In order to select the number and capacity of pumping units for the best economy, a comparison of the costs of different combinations of units should be made and the most economical combination determined by trial. The principles outlined in the preceding articles should be observed in making these comparisons. In a steam pumping station, when the number of units operating is less than the average daily maximum for the period, steam must nevertheless be kept on a sufficient number of boilers to operate the maximum number of pumps. This, and corresponding standby losses must not be overlooked, as they may show that a smaller number of larger units is ultimately more economical.

TABLE 33
Summary of Fluctuations of Sewage Flow at a Proposed Pumping Station
Number of Days Loads Occurred in One Year	Flow in Thousand Gallons per Minute	Lift in Feet	Horse-power
1	293	6.0	450
8	163	8.6	354
15	119	10.0	300
18	106	10.6	284
23	88	11.2	249
31	69	12.2	211
32	65	12.4	204
45	51	13.4	173
41	50	13.5	169
30	45	13.8	158
28	44	13.9	154
23	40	14.2	143
21	38	14.4	137
18	35	14.6	129
12	29	15.0	111
8	24	15.6	95
5	20	16.0	79
3	16	16.5	65
2	14	16.8	58
1	6.5	18.0	29
Total horse-power days for one year, 102,000.
Average load in horse-power, 280.

TABLE 34
Possible Combinations of Five Pumping Units to Care for the Loads Shown in Table 33[[50]]
40 Horse-power Type 1[[51]]				50 Horse-power Type 1[[51]]				60 Horse-power Type 1[[51]]				100 Horse-power Type 4[[51]]				200 Horse-power Type 5[[51]]				Load
Per Cent of Rated Capacity	Pounds Steam per H.P. Hour	Load in Horse-power	Pounds Steam, Units 10,000 Pounds	Per Cent of Rated Capacity	Pounds Steam per H.P. Hour	Load in Horse-power	Pounds Steam, Units 10,000 Pounds	Per Cent of Rated Capacity	Pounds Steam per H.P. Hour	Load in Horse-power	Pounds Steam, Units 10,000 Pounds	Per Cent of Rated Capacity	Pounds Steam per H.P. Hour	Load in Horse-power	Pounds Steam, Units 10,000 Pounds	Per Cent of Rated Capacity	Pounds Steam per H.P. Hour	Load in Horse-power	Pounds Steam, Units 10,000 Pounds	Number of Days Load is Carried in Year	Total Load Carried on these Days in H.P.
151	45	60.4	6.5	151	45	75.5	8.2	151	45	90.6	9.8	151	28	151	10.2	151	23	302	16.7	1	681
120	44	48	40.5	120	44	60.0	50.7	120	44	72.0	60.8	120	25	120	57.5	120	20	240	92.0	8	542
102	45	40.8	66.1	102	45	51.0	82.7	102	45	61.2	99.2	102	25	102	62.5	102	20	204	147	15	458
96	45	38.4	74.8	90	45	48.0	93.5	96	45	57.6	112	96	25	96	103.8	96	20	192	166	18	434
98	45	39.2	97.5	98	45	49.0	122.0					98	25	98	135.1	98	20	196	216	23	381
				104	45	52.0	174.5	104	45	62.4	209.0					104	20	208	309.5	31	322
				101	45	50.5	174.8	101	45	60.6	210					101	20	202	310	32	312
								102	45	61.2	325					102	20	204	481	45	264
				103	45	51.5	228									103	20	206	405	41	258
101	45	40.4	131													101	20	202	291	30	242
98	45	39.2	119													98	20	196	264	28	235
																109	20	218	241	23	218
																105	20	210	212	21	210
																99	20	198	171	18	198
								106	45	63.6	137	106	25	106	76.5					12	170
104	45	41.6	20.9									104	25	104	29.1					8	145
				109	44	54.5	28.8	109	44	65.4	34.5									5	121
												100	25	100	32.4					3	100
99	45	39.6	8.5	99	45	49.5	10.7													2	89
113	44	45.2	4.8																	1	45
Sub-total			596.6				973.9				1197.3				507.1				3322.2
Grand total in pounds, 65,700,000

TABLE 35
Financial Comparison of Pumping Equipments
The loads to be cared for are shown in Table 34. An emergency unit is supplied to bring the overload capacity of the plant, less the largest unit, equal to the maximum load on the plant. No unit will be overloaded more than fifty per cent of its rated capacity.
Number of Units Exclusive of Emergency Unit		5	4	3	2	1
Capacity and Type of Units		40 h.p., Type 1 50 h.p., Type 1 60 h.p., Type 1 100 h.p., Type 4 200 h.p., Type 5	50 h.p., Type 1 100 h.p., Type 4 125 h.p., Type 4 175 h.p., Type 5	50 h.p., Type 1 150 h.p., Type 5 250 h.p., Type 6	200 h.p., Type 5 250 h.p., Type 6	450 h.p., Type 7
Emergency Unit, Capacity and Type		200 h.p., Type 5	175 h.p., Type 5	250 h.p., Type 6	250 h.p., Type 6	450 h.p., Type 7
Annual payments, Dollars
	First cost of pumps	1,560	1,660	1,480	1,440	1,500
	Renewal of pumps	1,340	1,430	1,270	1,240	1,290
	First cost, boilers	1,024	1,089	1,125	1,115	1,410
	Renewal, boilers	800	935	966	958	1,210
	Fuel	13,140	11,860	10,490	9,420	9,400
	Repairs, oil, etc.	2,000	1,800	1,500	1,300	1,200
	Labor	35,000	31,500	29,500	27,000	27,000
	Emergency unit. First cost	640	560	800	800	1,500
	Emergency unit. Renewal	550	480	690	690	1,290
Total		56,134	51,314	47,821	43,963	45,800

Type 1. Simple duplex, non-condensing, horizontal. Type 4. Compound condensing low duty horizontal. Type 5. Low duty, triple, condensing, horizontal. Type 6. Cross compound, condensing, horizontal. Type 7. High duty, triple, condensing, vertical.

For example, the sewage flow expected at a proposed pumping station is shown in Table 33. The steps involved in the selection of the number and capacity of pumping units to care for these quantities are as follows: (1) Determine the rated capacity of the equipment to be provided. In this case the capacity will be taken as 450 horse-power, which is the maximum load to be placed on the pumps. (2) Select any number of units of such different types and capacities as are available for comparison, and arrange them in different combinations so that each unit will operate as nearly as possible at its rated capacity. The work involved in such a study for 5 units is shown in Table 34. The weight of steam consumed per indicated horse-power hour corresponding to the per cent of the rated capacity at which the unit is operating is read from Fig. 64 or other data. (3) Repeat this step for other numbers and types of units. (4) Prepare a table showing the annual costs of combinations of different numbers and types of units as shown for this example in Table 35. The figures in Table 35 show that the least expensive of the combinations of the units studied is one 200 horse-power unit, and one 250 horse-power unit, with a 250 horse-power unit in reserve. It is to be noted that a reserve unit has been provided in each combination, the capacity of which is equal to that of the largest unit of the combination.

CHAPTER VIII
MATERIALS FOR SEWERS

90. Materials.—The materials most commonly used for the manufacture of sewer pipe are vitrified clay and concrete. Cast iron, steel, and wood are also used, but only under special conditions. For pipes built in the trench, concrete, concrete blocks, brick, and vitrified clay blocks are used. Concrete is being used to-day more than bricks or blocks because it is cheaper. A decade or more ago all large sewers were built of bricks. Vitrified clay and concrete are used for manufactured pipe 42 inches and less in diameter. Concrete is used almost exclusively for larger sizes of pipe, particularly for pipe constructed in place, although a brick invert lining is advisable when high velocities of flow are expected.

The character of the external load, the velocity of flow and the quality of sewage are important factors in determining the material to be used in the construction of sewers. Reinforced concrete should be used for large sewers near the surface subjected to heavy moving loads. A high velocity of flow with erosive suspended matter demand a brick wearing surface on the invert. Many engineers consider concrete less suitable than vitrified clay or brick for conveying septic sewage or acid industrial wastes, as concrete deteriorates more rapidly under such conditions. Concrete should be used on soft yielding foundations, whereas a hard compact earth, which can be cut to the form of the sewer, is suitable to the use of brick or concrete.

Cast-iron pipe with lead joints is used for sewers flowing under pressure, or where movements of the soil are to be expected. If the sewage is not flowing under pressure, cement joints are sometimes used in the cast-iron pipe. Movements of the soil are to be expected on side hills, under railroad tracks, etc. Steel pipe is used on long outfalls or under other conditions where external loads are light and the cost is less than for other materials. Because of the thin plates used and the liability to corrosion steel is not frequently used. It should never be deeply buried nor externally loaded because of its weakness in resisting such forces. Like wood pipe, its lightness is favorable to use on bridges, but the greater heat conductivity of steel than wood necessitates protection against freezing in exposed positions. Wood is preferable only where the economy of its use is pronounced and the pipe is running full at all times. It is desirable that the wood pipe should be always submerged as the life of alternately wet and dry wood is short.

Corrugated galvanized iron and unglazed tile have been used for sewers, but usually only in emergencies or as a makeshift. Corrugated iron is not suitable on account of its roughness and liability to corrosion, and unglazed tile because of its lack of strength.

Fig. 71.—Diagrammatic Section through Clay-pipe Press.

91. Vitrified Clay Pipe.—In general the physical and chemical qualities of clays before burning are not sufficient to cause their condemnation or approval by the engineer, as their behavior in the furnace is quite individual and depends greatly on the manner in which they are fired. The engineer is interested in the result and writes his specifications accordingly.

In the manufacture of clay pipe, the clay as excavated is taken to a mill and ground while dry, to as fine a condition as possible. It is then sent to storage bins from which it is taken for wet grinding and tempering. In this process the clay is mixed with water to the proper degree of plasticity. A variation of 1 to 1½ per cent in the moisture content will mean failure. Too wet a mixture will not have sufficient strength to maintain its shape in the kiln. Too dry a mixture will show laminations as it is pressed through the discs.

A press used in the manufacture of clay pipe is shown in cross-section in Fig. 71. With the piston heads in the steam and mud cylinders at their extreme upward positions, the mud cylinder is filled with clay of the proper consistency. Steam is then turned into the steam cylinder under pressure and the clay is squeezed into the space between the inner and outer shells of the die and mandrel to form the hub of the pipe. The pressure on the clay may be from 250 to 600 pounds per square inch. When clay appears at the holes, marked hh at the bottom of the mud cylinder, the bottom plate and the center portion of the die are removed and the remainder or straight portion of the pipe is formed by squeezing the clay between the mandrel and the outer wall of the die. A completely formed pipe can be seen issuing from the press in Fig. 72. Any sized pipe that is desired can be formed from the same press by changing the size of the dies and mandrel.

Fig. 72.—Clay-pipe Press.
Courtesy, Blackmer and Post Manufacturing Co.

Curved pipes are made in two ways—by bending directly as they issue from the press, or by shaping by hand in plaster of paris molds. Junctions are made by cutting the branch pipe to the shape of the outside of the main pipe, fastening the branch in place with soft clay and then cutting out the wall of the main pipe the size of the branch. Special fittings are usually made by hand in plaster molds.

After being pressed into shape the pipes are taken to a steam-heated drying room where a constant temperature is maintained in order to prevent cracking of the pipes. They remain in the drying room from 3 to 10 days until dry, when they are taken to the kilns. If taken to the kilns when moist blisters will be produced.

The dried pipes are piled carefully in the kiln so that heat and weight may be as evenly distributed as possible, and the fire is then started in the kiln. The process of burning can be roughly divided into five stages:

1st. Water smoking, which lasts about 72 hours during which the temperature is raised gradually to 350 degrees Fahrenheit.

2nd. Heating, during which the temperature is raised to 800 degrees Fahrenheit in 24 hours.

3rd. Oxidation, during which the temperature is raised to 1,400 degrees Fahrenheit in 84 hours.

4th. Vitrification, in which the temperature is raised to 2,100 degrees Fahrenheit in 48 hours, and finally,

5th. Glazing, during which the temperature is unchanged but salt (NaCl) is thrown in and allowed to burn.

Oxidation must be complete before vitrification is started as otherwise blisters will be raised due to imprisoned carbon dioxide. The important points in vitrification are to make the required temperature within a reasonable time and to maintain a uniform distribution of heat throughout the kiln. When vitrification is complete as shown by a glassy fracture of a broken sample taken from the kiln, glazing is accomplished by throwing a shovelful of salt on the hottest part of the fire. About five to six applications of salt from two to three hours apart may be needed. The kiln is then allowed to cool and the manufacture of the pipe is complete. The completeness of vitrification is indicated by the amount of water that the finished pipe will absorb. Completely vitrified pipe will absorb no moisture. Soft-burned pipe may absorb as much as 15 per cent moisture.

Vitrified clay blocks are made of the same material and in the same manner as vitrified clay pipe.

The following data on vitrified pipe have been abstracted from the specifications for vitrified pipe adopted by the American Society for Testing Materials.

Pipes shall be subject to rejection on account of the following:

(a) Variation in any dimension exceeding the permissible variations given in Table 36.

(b) Fracture or cracks passing through the shell or hub, except that a single crack at either end of a pipe not exceeding 2 inches in length or a single fracture in the hub not exceeding 3 inches in width nor 2 inches in length will not be deemed cause for rejection unless these defects exist in more than 5 per cent of the entire shipment or delivery.

(c) Blisters or where the glazing is broken or which exceed 3 inches in diameter, or which project more than ⅛ inch above the surface.

(d) Laminations which indicate extended voids in the pipe material.

(e) Fire cracks or hair cracks sufficient to impair the strength, durability or serviceability of the pipe.

(f) Variations of more than ⅛ inch per linear foot in alignment of a pipe intended to be straight.

(g) Glaze which does not fully cover and protect all parts of the shell and ends except those exempted in Sect. 31. Also glaze which is not equal to best salt glaze.

(h) Failure to give a clear ringing sound when placed on end and dry tapped with a light hammer.

(i) Insecure attachment of branches or spurs.

Workmanship and Finish

(29) Pipes shall be substantially free from fractures, large or deep cracks and blisters, laminations and surface roughness.

(31) The glaze shall consist of a continuous layer of bright or semi-bright glass substantially free from coarse blisters and pimples.... Not more than 10 per cent of the inner surface of any pipe barrel shall be bare of glaze except the hub, where it may be entirely absent. Glazing will not be required on the outer surface of the barrel at the spigot end for a distance from the end equal to ⅔ the specified depth of the socket for the corresponding size of pipe. Where glazing is required there shall be absence of any well defined network of crazing lines or hair cracks.

(32) The ends of the pipe shall be square with their longitudinal axis.

(33) Special shapes shall have a plain spigot end and a hub end corresponding in all respects with the dimensions specified for pipes of the corresponding internal diameter.

TABLE 36
Properties of Clay Sewer Pipe
Abstracts from Tentative Specifications of the American Society for Testing Materials
Internal Diameter, Inches	Minimum Crushing Strength, Pounds per Linear Foot. See Note 2	Maximum Absorption, Per Cent	Laying length, Feet	Diameter of Inside of Socket, Inches	Depth of Socket Inches	Taper of Socket	Minimum Thickness of Barrel. Inches	Permissible Variations						Number of Scorings on Spigot and Socket ⅛ Inch Deep
								Length, Inches (-), per Foot	Internal Diameter, Inches		Length of Two Opposite Sides, Inches	Depth of Socket, Inches (-)	Thickness of Barrel, Inches (-)
									Spigot (±)	Socket (±)
6	1430	5	2, 2½, 3	8¼	2	1 : 20	⅝	¼	3 16	¼	⅛	¼	1 16	2
8	1430	5	2, 2½, 3	10¾	2¼	1 : 20	¾	¼	¼	5 16	⅛	¼	1 16	2
10	1570	5	2, 2½, 3	13	2½	1 : 20	⅞	¼	¼	5 16	⅛	¼	1 16	2
12	1710	5	2, 2½, 3	15¼	2½	1 : 20	1	¼	5 16	⅜	⅛	¼	1 16	2
15	1960	5	2, 2½, 3	18¾	2½	1 : 20	1¼	¼	5 16	⅜	⅛	¼	3 32	3
18	2200	5	2, 2½, 3	22¼	3	1 : 20	1½	¼	⅜	7 16	3 16	¼	3 32	3
21	2590	5	2, 2½, 3	26	3	1 : 20	1¾	¼	7 16	½	3 16	¼	⅛	3
24	3070	5	2, 2½, 3	29½	3	1 : 20	2	⅜	½	9 16	¼	¼	⅛	4
27	3370	5	3	33¼	3½	1 : 20	2¼	⅜	⅝	11 16	¼	¼	⅛	4
30	3690	5	3	37	3½	1 : 20	2½	⅜	⅝	11 16	¼	¼	⅛	4
33	3930	5	3	40¼	4	1 : 20	2⅝	⅜	¾	13 16	¼	¼	3 16	5
36	4400	5	3	44	4	1 : 20	2¾	⅜	¾	13 16	⅜	¼	3 16	5
39	4710	5	3	47¼	4	1 : 20	2⅞	⅜	¾	13 16	⅜	¼	3 16	5
42	5030	5	3	51	4	1 : 20	3	⅜	¾	13 16	⅜	¼	3 16	5

Note 1. For methods of making tests see Proc. Am. Soc. for Testing Materials.

Note 2. Concentrated load at end of vertical diameter.

(a) Slants shall have their spigot ends cut at an angle of approximately 45 degrees with the longitudinal axis.

(b) Curves shall be at angles of 90, 45, 22½, and 11¼ degrees as required. They shall conform substantially to the curvature specified.

(c) ... All branches shall terminate in sockets.

Fig. 73.—Standard Clay Pipe Specials.
Courtesy, Blackmer and Post Manufacturing Co.

In Fig. 73 are shown the various forms of vitrified pipe and specials which are ordinarily available on the market.

The life of vitrified clay sewers and some observations on the results of the inspection of the sewers in Manhattan are discussed in Chapter XII. The strength of vitrified sewer pipes is shown in Table 37.

TABLE 37
Strength of Sewer Pipe
Strength in pounds per linear foot to carry loads from ditch filling material such as ordinary sand and thoroughly wet clay, with the under side of the pipe bedded 60° to 90° by ordinary good methods. From Proc. Am. Society for Testing Materials, Vol. 20, 1920, page 604.
Height of Fill Above Top of Pipe, Feet	Breadth of the Ditch a Little Below the Top of the Pipe
	1 Foot		2 Feet		3 Feet		4 Feet		5 Feet
	Ditch Filling Material
	sand	clay	sand	clay	sand	clay	sand	clay	sand	clay
2	265	280	615	635	970	990	1330	1,350	1,690	1,710
4	400	450	1055	1125	1745	1825	2455	2,535	3,165	3,250
6	470	545	1370	1500	2370	2525	3405	3,575	4,460	4,740
8	505	605	1600	1790	2875	3115	4215	4,495	5,595	5,890
10	525	640	1765	2015	3275	3610	4900	5,295	6,590	7,020
12	535	660	1880	2185	3600	4030	5485	6,000	7,460	8,035
14	540	675	1965	2320	3855	4380	5975	6,620	8,225	8,950
16	545	680	2025	2425	4065	4675	6395	7,165	8,890	9,775
18	545	685	2070	2505	4230	4920	6750	7,630	9,480	10,520
20	545	690	2100	2565	4365	5130	7050	8,060	9,995	11,190
22	545	690	2125	2610	4470	5305	7305	8,425	10,445	11,795
24	545	690	2140	2645	4560	5445	7525	8,750	10,840	12,340
26	545	690	2150	2675	4630	5575	7705	9,035	11,185	12,830
28	545	690	2160	2695	4685	5680	7860	9,280	11,490	13,270
30	545	690	2165	2715	4725	5765	7990	9,500	11,755	13,670
Very great	545	690	2180	2770	4910	6230	8725	11,075	13,635	17,305

92. Cement and Concrete Pipe.—Although there is no general recognition of a difference between cement and concrete pipe, there is a tendency to term manufactured pipe of small diameter cement pipe, and large pipes or pipes constructed in place, concrete pipe. Cement, unlike clay, is used in the manufacture of pipe in the field or by more or less unskilled operators in “one man” plants. Great care should be used in the selection of cement, aggregate, and reinforcement for precast cement pipe since the shocks to which it is subjected in transit are more liable to rupture it than the heavier but steadier loads imposed on it in the trench.

The United States Government, various scientific and engineering societies, and other interested organizations have collaborated in the preparation of specifications for cement and cement tests. These specifications can be found in Trans. Am. Soc. Civil Engineers, Vol. 82, 1918, p. 166, and in other publications.

The following abstracts have been taken from the proposed tentative specifications for Concrete Aggregates, of the Am. Society for Testing Materials, issued June 21, 1921:

1. Fine aggregate shall consist of sand, stone screenings, or other inert materials with similar characteristics, or a combination thereof, having clean, hard, strong, durable uncoated grains, free from injurious amounts of dust, lumps, soft or flaky particles, shale, alkali, organic matter, loam or other deleterious substances.

2. Fine aggregates shall preferably be graded from fine to coarse, with the coarser particles predominating, within the following limits:

Passing No. 4 sieve	100 per cent
Passing No. 50 sieve, not more than	50 per cent
Weight removed by elutriation test, not more than	3 per cent

Sieves shall conform to the U. S. Bureau of Standards specifications for sieves.

3. The fine aggregate shall be tested in combination with the coarse aggregate and the cement with which it is to be used and in the proportions, including water, in which they are to be used on the work, in accordance with the requirements specified in Section 6....

7. Coarse aggregate shall consist of crushed stone, gravel or other approved inert materials with similar characteristics, or a combination thereof, having clean, hard, strong, durable, uncoated pieces free from injurious amounts of soft, friable, thin, elongated or laminated pieces, alkali, organic or other deleterious matter.

---

The following Table indicates desirable gradings, in percentages, for coarse aggregate for certain maximum sizes.

Gradings of Coarse Aggregates
Maximum Size of Aggregate Inches	Circular Openings, Inches								Passing Screen Having Circular Openings ¼ Inch in diameter, not more than
	3	2½	2	1½	1¼	1	¾	½
3	100			40–75					15 per cent
2½		100			40–75				15 per cent
2			100			40–75			15 per cent
1½				100			40–75		15 per cent
1¼					100			35–70	15 per cent
1						100		40–75	15 per cent
¾							100		15 per cent

The manufacture of small size cement pipe requires relatively more skill than equipment. As a result great care must be observed in the inspection of cement pipe and in the enforcement of specifications. For large size concrete pipe and reinforced concrete pipe the difficulty of holding the pipe together during transportation and lowering into the trench aid in insuring a good product.

Cement pipe is made by ramming a mixture of cement, sand, and water into a cylindrical mold and allowing it to stand until set. The mold is then removed and the pipe stands for a further period of time to become cured. The selection and proportion of materials, the amount of water, the method of ramming, the period of setting, the length of time of curing, and the control of moisture and temperature during this period are of great importance in the resulting product. E. S. Hanson[^[52]] states that the most conservative engineers recommend a mixture of one sack of cement to 2½ cubic feet of aggregate measured as loosely thrown into the measuring box. In making up the aggregate, clean gravel or broken stone up to ¼ inch in size is used. The American Concrete Institute recommends that 100 per cent pass a ½-inch screen, 70 per cent a ¼-inch screen, 50 per cent a No. 10, 40 per cent a No. 20, 30 per cent a No. 30, and 20 per cent a No. 40. The materials should be carefully graded by experiment and not guessed at, as the behavior of all aggregates is not the same. Too coarse an aggregate is difficult to handle in manufacturing. It causes loss of pipe when the jacket or mold is removed and results in rough pipe, stone pockets, and pin holes through which water spurts when pressure tests are applied. Too fine an aggregate causes loss of strength and with ordinary mixtures tends to produce a pipe which will show seepage under internal pressure tests. The amount of water in the mixture will vary, from 15 to 20 per cent. The mixture should appear dry but should ball in the hand under some pressure.

Fig. 74.—Details of 24–Inch Concrete Pipe Form.

The mixture can be rammed into the molds by hand or machine. A machine-made pipe is preferable as it produces a more even and stronger product. There are two types of machines for this purpose. One type consists of a number of tamping feet which deliver about 200 blows to the minute with a pressure of about 800 pounds per square inch of area exposed. In the other type a revolving core is drawn through the pipe, packing and polishing the concrete as it is pulled through, with special provision for packing the bell of the pipe. The tamping machines can make 1,500 feet of small size pipe to 300 feet of 24–inch pipe in a day. Machines of the second type can make 750 feet of 8–inch to 200 feet of 30–inch pipe in 30–inch lengths in 9 hours. The inside and outside forms for a 24–inch pipe are shown in Fig. 74 as used with the tamping machines. The forms are swabbed with oil before being filled in order to facilitate their removal. In making a Y-branch or other special, a hole is cut in the pipe or mold the size of the joining pipe which is then set in place and the joint wiped smooth with cement.

TABLE 38
Properties of Cement Concrete Sewer Pipe
1917 Specifications of American Society for Testing Materials, with Subsequent Revisions
Internal Diameter, Inches	Laying Length, Feet	Diameter at Inside of Socket, Inches	Normal Annular Space, Inches	Depth of Socket, Inches	Taper of Socket	Minimum Thickness of Barrel, Inches	Limits of Permissible Variations					Minimum Crushing Strength, Pounds per Linear Foot at End of Diameter	Maximum Absorption, Per Cent
							Length, Inch per Foot (-)	Internal Diameter, Inches		Depth of Hub (-) Inches	Thickness of Barrel (-) Inches
								Spigot (±)	Socket (±)
6	2, 2½, 3	8¼	½	2	1 : 20	⅝	¼	3 16	3 16	¼	1 16	1430	8
8	2, 2½, 3	11	⅝	2¼	1 : 20	¾	¼	¼	¼	¼	1 16	1430	8
10	2, 2½, 3	13¼	⅝	2½	1 : 20	⅞	¼	¼	¼	¼	1 16	1570	8
12	2, 2½, 3	15⅝	⅝	2½	1 : 20	1	¼	¼	¼	¼	1 16	1910	8
15	2, 2½, 3	19¼	⅝	2½	1 : 20	1¼	¼	¼	¼	¼	3 32	1960	8
18	2, 2½, 3	22¾	⅝	2¾	1 : 20	1½	¼	¼	¼	¼	3 32	2200	8
21	2, 2½, 3	26½	¾	2¾	1 : 20	1¾	¼	5 16	5 16	¼	⅛	2590	8
24	2, 2½, 3	30¼	¾	3	1 : 20	2⅛	⅜	5 16	5 16	¼	⅛	3070	8
27	3	34	⅞	3¼	1 : 20	2¼	⅜	5 16	⅜	¼	⅛	3370	8
30	3	38	1	3½	1 : 20	2½	⅜	⅜	⅜	¼	⅛	3690	8
33	3	41½	1	4	1 : 20	2¾	⅜	⅜	⅜	¼	3 16	3930	8
36	3	45½	1¼	4	1 : 20	3	⅜	½	½	¼	3 16	4400	8
39	3	49	1¼	4	1 : 20	3¼	⅜	½	½	¼	3 16	4710	8
42	3	53	1½	4	1 : 20	3½	⅜	½	½	¼	3 16	5030	8

After the removal of the mold the pipe may be cured by the water or the steam process. Hanson states:

By the former the pipe are simply set on the floor of the plant and as soon as they are sufficiently strong so that they can be sprinkled with water without falling down; sprinkling is commenced and continued at such intervals for 6 or 7 days that the pipe will be moist at all times. This is a slower process than steam curing. It is also less uniform and less subject to control than where the product is cured by steam.

In the steam process the pipe is exposed to low-pressure steam with plenty of moisture in a closed receptacle for 24 hours, or until hardened. It has been found by tests that pipes sprinkled for 28 days are as strong as steam-cured pipes.

The dimensions of cement concrete sewer pipe as recommended by the Am. Society for Testing Materials are shown in Table 38.

The following has been abstracted from the description of the manufacture of one form of concrete pipe by G. C. Bartram.[^[53]] All pipe are manufactured in 4–foot lengths near the site at which they are to be installed because of their great weight, for example, 36–inch pipe weighs one ton. The plant for the manufacture of the pipe consists of cast-iron bottom and top rings for each size to be used on the job, and inside and outside steel casings. There are three bases for each steel casing as the pipes stand on the bases for 72 hours and the steel casing remains on for only 24 hours after the concrete has been poured. The pipes are then lifted off the bases and stored for aging. The pipes are cast with the spigot end up.

The concrete is ordinarily mixed in the proportions of 1 : 2 : 4. The materials are placed in the mixer in the following order: first, the stone, then the sand, then the cement, and finally the water. Sufficient water is added to make the concrete flow freely. In cold weather or for a hurry-up job the molds are covered with canvas and are steamed for 2 or 3 hours immediately after the concrete is poured. The molds are then removed but the pipe should be steamed before use. Otherwise they are allowed to stand 72 hours, as explained above. In cold weather the steam is used to prevent freezing and not to hasten the completion of the pipe.

Fig. 75.—Triangle Mesh Reinforced Concrete Pipe.
As made by the Am. Concrete Pipe and Pile Co., Chicago.

Fig. 76.—Methods of Joining and Reinforcing Concrete Pipe.

One layer or ring of reinforcement is used for sizes from 24 to 48 inches and two layers or rings for larger pipe. A type of reinforcement sometimes used is the American Steel and Wire Company’s Triangular Mesh, an illustration of which is shown in Fig. 75. The wire mesh is cut to fit and is placed in a slot in the cast-iron base. The slot is then filled with sand so that the concrete cannot enter, thus leaving a portion of the reinforcement exposed. The inside reinforcement extends through and out of the spigot of the completed pipe. In the trench the two reinforcements overlap in the key-shaped space left on the inside of the pipe by the design of the bell and spigot. This space is shown in Fig. 76 A. When the pipe is placed in the trench the key-shaped space is plastered with mortar and a piece is knocked out of the bell to receive the grout with which the joint is closed. A spring steel band is then put on the outside of the joint and grout poured into the hole at the top. The band is removed as soon as the joint materials have set.

The rules for the reinforcement of concrete pipe recommended in Volume XV, 1919, of the Transactions of the Concrete Institute are as follows:

No reinforcement is approved for pipe between 30 and 60 inches in diameter or in rock or hard soils. For pipe 36 inches in diameter or less the minimum thickness of shell shall be 5 inches. For 60–inch pipe the minimum thickness shall be 7 inches with intermediate sizes in proportion. Reinforcement for circular pipe shall consist of one or two rings of circular wire fabric or rods of the areas shown in Table 39. All sewers near the surface and subject to vibration should be reinforced. For sewers 6 feet or less in diameter the reinforcement should consist of at least ½ of 1 per cent of the area of the concrete. It should be placed near the inside at the crown and near the outside at the haunches. If large horizontal pressures are expected the pipe should be reinforced for these reverse stresses, which involves placing the reinforcement near the outside at the crown and near the inside at the haunches. The minimum thickness of the walls of sewers greater than 6 feet in diameter with flat bottom and arch, with or without side walls, should be 8 inches.

TABLE 39
Reinforcement for Circular Concrete Sewer Pipe
(See Vol. XV, Proceedings Am. Concrete Institute)
Diameter in Inches	Minimum Thickness of Shell in Inches	Number of Rings	Cross Sectional Area of Each Ring
24	3	1	.058
27	3	1	.068
30	3½	1	.080
33	4	1	.107
36	4	1	.146
39	4	1	.146
42	4½	1	.153
48	5	2	.107
54	5½	2	.123
60	6	2	.146
66	6½	2	.168
72	7	2	.180
84	8	2	.208
96	9	2	.245

Three methods for the reinforcement of concrete sewers are shown in Fig. 76 B.

93. Proportioning of Concrete.—In the proportioning of concrete questions of strength, of permeability, and of workability[^[54]] may need consideration. All of these qualities are affected by the amount of cement, the nature and gradation and relative proportions of the fine and the coarse aggregate, and the amount of mixing water used.

Other things being equal the strength varies with the amount of cement put into the concrete. For the same amount of cement and the same consistency of the mixture, the strength increases with increased density of concrete (that is, with decreased voids), and the effort should be made so to proportion the fine and coarse aggregates as to produce the densest concrete (least voids) with the aggregates available. For the same consistency, the strength then will vary with the ratio of the amount of cement to the amount of the voids.

So far as the mixing water is concerned, the greatest strength in the concrete will be attained at a rather dry mix; that which produces the least volume of concrete. The addition of more water results in a concrete of less strength; 40 per cent more water may give a concrete of less than half the normal strength. The reduction in strength is then very marked for the wetter mixes, and the water content used is a feature of considerable importance in the design of concrete mixtures.

Permeability is affected by the same elements as strength, but the size and discontinuity of the pores have a greater influence.

Workability is an important quality; in some respects it will have to be obtained at the expense of strength. Increasing the amount of mixing water increases the workability of the mixtures, with a resulting decrease in strength which may have to be accepted or else overcome by increasing the cement in the mix.

An excess of water is often used unnecessarily through ignorance of the injurious results. A high proportion of coarse aggregate, up to a certain limit, will give concrete of high strength, but the mixture will be harsh-working and not easy to place. Lower proportions of coarse aggregate will give greater workability and better uniformity of product, the latter being an important matter. It is apparent that the degree of workability of the mixture needed will depend upon the nature of the construction—for a pavement where the concrete will receive substantial tamping or working the water content may be much less than that which may need to be used in placing concrete around reinforcement in narrow members, or where little tamping or spading can be done. The nature of the work will affect the standard of consistency to be specified.

The proportioning of the concrete should then be dependent upon the needs of the structure and the manner of placing the concrete. The proportions selected should be carefully adhered to and especially should care be taken to see that the right quantity of mixing water is used.

The materials are commonly measured volumetrically (by bulk). Because of the variations which are introduced by volumetric measurement of the materials by the presence of varying degrees of moisture, measurements by weight would be more accurate, but these would also be affected by differences in the specific gravity of the materials. The methods of measuring, the allowance for moisture, as well as the proportions of the materials, should be specified.

The methods for proportioning concrete are:

(1) Arbitrarily selected proportions.

(2) Proportions based on minimum voids.

(3) Proportions based on trial mixtures.

(4) Proportions based on a sieve analysis curve.

(5) Proportions based on the surface area of the aggregates.

(6) Proportions based on the water-cement ratio and the fineness modulus.

(7) Proportions based on mortar-voids and cement-voids ratio.

Arbitrarily selected proportions are in quite general use; they are intended to apply to the materials most commonly used in the vicinity of the work. The most common practice is to use twice as great a volume of coarse aggregate as fine aggregate, as for instance 1 part cement, 2 parts fine aggregate, and 4 parts coarse aggregate. Decreasing the ratio of coarse aggregate to fine aggregate may give a more easily worked mix or require relatively less water for a given workability, and in some cases it will be proper to increase this ratio and thus secure an increase of strength. Judgment and experience with given materials may warrant changes from a stated ratio. The proportions are now frequently given as one part cement to a certain number of parts of the mixed aggregate, leaving the proportions of the fine to coarse to be determined otherwise, since small variations in the relation of these will not greatly affect the strength. Proportions in common use are:[^[55]]

Mortar for
	Laying brick and stone masonry	from	1 : 0 to 1 : 3
	Filling joints in sewer pipe		1 : 0 to 1 : 2
	Surfaces, floors, sidewalks, pavements		1 : 0 to 1 : 2
	Waterproof linings		1 : 0 to 1 : 2
	Cement, bricks, and blocks		1 : 2½ to 1 : 4
Concrete for
	Gravity retaining walls, heavy foundations, structures needing mass more than strength	from	1 : 3 : 6 to 1 : 4 : 8
	Retaining walls, piers, sewers, pavements, foundations, and work requiring strength. (Compressive strength in 28 days, 1,500 to 2,000 pounds per square inch)	from	1 : 2 : 4 to 1 : 3 : 6
	Floors, beams, pavements, reinforced concrete, arch bridges, low-pressure tanks. (Compressive strength in 28 days, 2,000 to 3,000 pounds per square inch)	from	1 : 1½ : 3 to 1 : 2½ : 4½
	Reinforced concrete columns, conduit pipe, impervious concrete. (Compressive strength in 28 days, 3,000 to 4,000 pounds per square inch)	from	1 : 1 : 2 to 1 : 1½ : 3

The usual method of proportioning based on minimum voids is to assume that the particles of fine aggregate should fill the voids in the coarse aggregate and that the particles of the cement will fill the voids in the fine aggregate. About 5 to 10 per cent additional fine aggregate is generally added to push the particles of the coarse aggregate apart and thus give a more easily worked concrete and one freer from void spaces. This method is inaccurate, principally because of the effect of the moisture on the volume of the voids, and because the effect on the volume by the addition of water is unknown.

Trial mixtures may be made by carefully weighing each of the ingredients and then combining them to give a workable concrete. Using a given amount of cement, the proportion of ingredients, of the same total weight, which will give the least volume and therefore the densest concrete is adopted. When making the comparison the consistency of the mixes must be maintained constant.

Proportioning may be based on an ideal sieve analysis curve of the mixed cement and aggregates. The sieve analysis of the aggregates is made by screening a predetermined weight of the sample through a series of 5 to 8 sieves graded in size from slightly below the size of the largest particle to slightly above the smallest particle of the aggregate. The analysis is then expressed in the form of a curve. The ideal curve, according to Fuller,[^[56]] is shown in Fig. 77.

Fig. 77.—Gravel Analysis.
The dotted line indicates the ideal combination of the coarse and fine portions. The heavy full line indicates the combination attained.

The method of proportioning concrete by surface areas is based on the theory that the strength of a concrete depends on the amount of cement used in proportion to the surface area of the aggregates.[^[57]]

The proportioning of concrete on the basis of a water-cement ratio and a fineness modulus was introduced by Prof. D. A. Abrams.[^[58]] It is based on the theory that with fixed conditions of aggregate, moisture, etc., the ratio of water to cement determines the strength of the concrete.

A method of proportioning concrete by determining experimentally the voids in mortars made up with a given amount of sand and definite proportions of cement, and then calculating the voids in the concrete made up by adding a definite amount of coarse aggregate to the mixture, has been developed.[^[59]] The method is based on the theory that the strength of the concrete is a known function of the ratio of the volume of cement to the volume of the voids in the concrete. The effect of varying the proportion of the ingredients, including an increase in the amount of mixing water beyond that required to give the densest mixture, may be found by the method, and a comparison may be made of results obtainable with different classes of fine and coarse aggregates.

Arbitrarily selected proportions, proportions based on voids, and proportions based on trial mixtures are usually satisfactory for small jobs where the amount of materials involved is not large. Where the saving in materials will permit, more accurate methods should be used. The methods can be studied more fully by reference to the original articles quoted in the footnotes, or to the following texts:

Materials of Construction, Johnson, 5th Edition, 1918.

Materials of Engineering, H. F. Moore, 2d Edition, 1920.

Masonry Construction, I. O. Baker, 10th Edition, 1912.

Concrete Engineer’s Handbook, Hool and Johnson, 1918.

Concrete, Plain and Reinforced, Taylor and Thompson, 1916.

94. Waterproofing Concrete.—The waterproofing of concrete is most satisfactorily done by making dense mixtures. In practice such substances as hydrated lime, clay, alum and soap, and proprietary compounds such as Ceresit, Medusa, etc., are frequently mixed with the concrete under the theory that these very fine substances will fill any remaining voids and render the concrete impervious. The specifications of the Joint Committee issued on June 4, 1921, are much briefer and contain less detailed instruction than those issued earlier.[^[60]] The earlier instructions follow.

Many expedients have been resorted to for making concrete impervious to water. Experience shows, however, that when mortar or concrete is proportioned to obtain the greatest practicable density and is mixed to the proper consistency, the resulting mortar or concrete is impervious under moderate pressure.

On the other hand concrete of dry consistency is more or less pervious to water, and, though compounds of various kinds have been mixed with the concrete or applied as a wash to the surface, in an effort to offset this defect, these expedients have generally been disappointing, for the reason that many of these compounds have at best but temporary value, and in time lose their power of imparting impermeability to the concrete.

In the case of subways, long retaining walls, and reservoirs, provided the concrete itself is impervious, cracks may be so reduced, by horizontal and vertical reinforcement properly proportioned and located, that they will be too minute to permit leakage, or will be closed by infiltration of silt.

Asphaltic or coal tar preparations applied either as a mastic or as a coating on felt cloth or fabric, are used for waterproofing, and should be proof against injury by liquids or gases.

For retaining and similar walls in direct contact with the earth, the application of one or two coatings of hot coal tar pitch, following a painting with a thin wash of coal tar dissolved in benzol, to the thoroughly dried surface of concrete is an efficient method of preventing the penetration of moisture from the earth.

Tar paper and asphaltic compounds are not often used in sewer work as absolute imperviousness is seldom necessary.

95. Mixing and Placing Concrete.—Careful workmanship is desirable in the mixing and placing of concrete in sewers since water-tight construction is desired. Because of the difficulty of inspecting concrete in wet, dark and crowded excavations, and the careless habits of workmen experienced in concrete sewer construction, the highest class of concrete work cannot be expected. The situation is met by designing thick walls as shown in the sections illustrated in Fig. 22 and 23.

In the report of the Joint Committee on Concrete and Reinforced Concrete in Transactions of the American Society of Civil Engineers for 1917, on page 1101 the recommendation is made concerning the mixing and placing of concrete as follows:[^[61]]

The mixing of concrete should be thorough and should continue until the mass is uniform in color and is homogeneous. As the maximum density and greatest strength of a given mixture depends largely on thorough and complete mixing, it is essential that this part of the work should receive special attention and care.

Inasmuch as it is difficult to determine by visual inspection whether the concrete is uniformly mixed, especially where aggregates having the color of cement are used, it is essential that the mixing should occupy a definite period of time. The minimum time will depend on whether the mixing is done by machine or hand.

(a) Measuring Ingredients: Methods of measurement of the various ingredients should be used which will secure at all times separate and uniform measurements of cement, fine aggregate, coarse aggregate and water.

(b) Machine Mixing: The mixing should be done in a batch machine mixer of a type which will insure the uniform distribution of the materials throughout the mass, and should continue for the minimum time of 1½ minutes after all the ingredients are assembled in the mixer. For mixers of 2 or more cubic yards capacity, the minimum time of mixing should be 2 minutes. Since the strength of the concrete is dependent on thorough mixing, a longer time than this minimum is preferable. It is desirable to have the mixer equipped with an attachment for automatically locking the discharging device so as to prevent the emptying of the mixer until all the materials have been mixed together for the minimum time required after they are assembled in the mixer. Means should be provided to prevent aggregates being added after the mixing has commenced. The mixer should also be equipped with water storage, and an automatic measuring device which can be locked if desired. It is also desirable to equip the mixer with a device recording the revolutions of the drum. The number of revolutions should be so regulated as to give at the periphery of the drum a uniform speed. About 200 feet per minute seems to be the best speed in the present state of the art.

(c) Hand Mixing: Hand mixing should be done on a water-tight platform and especial precautions taken after the water has been added, to turn all the ingredients together at least 6 times, and until the mass is homogeneous in appearance and color.

(d) Consistency: The materials should be mixed wet enough to produce a concrete of such a consistency as will flow sluggishly into the forms and about the metal reinforcement when used, and which at the same time can be conveyed from the mixer to the forms without separation of the coarse aggregate from the mortar. The quantity of water is of the greatest importance in securing concrete of maximum strength and density; too much water is as objectionable as too little.

(e) Retempering: The remixing of concrete and mortar that has partly reset should not be permitted.

Placing Concrete

(a) Methods: Concrete after the completion of the mixing should be conveyed rapidly to the place of final deposit; under no circumstances should concrete be used that has partly set.

Concrete should be deposited in such a manner as will permit the most thorough compacting such as can be obtained by working with a straight shovel or slicing tool kept moving up and down until all the ingredients are in their proper place. Special care should be exercised to prevent the formation of laitance; where laitance has formed it should be removed, since it lacks strength and prevents a proper bond in the concrete.

Care should be taken that the forms are substantial and thoroughly wetted (except in freezing weather) or oiled, and that the space to be occupied by the concrete is free from all debris. When the placing of concrete is suspended, all necessary grooves for joining future work should be made before the concrete has set.

When work is resumed concrete previously placed should be roughened, cleansed of foreign material and laitance, thoroughly wetted and then slushed with a mortar consisting of one part Portland cement and not more than 2 parts of fine aggregate.

The surfaces of concrete exposed to premature drying should be kept covered and wet for at least 7 days.

Where concrete is conveyed by spouting, the plant should be of such a size and design as to insure a practically continuous stream in the spout. The angle of the spout with the horizontal should be such as to allow the concrete to flow without separation of the ingredients; in general an angle of about 27 degrees or 1 vertical to 2 horizontal is good practice. The spout should be thoroughly flushed with water before and after each run. The delivery from the spout should be as close as possible from the point of deposit. Where the discharge must be intermittent, a hopper should be provided at the bottom. Spouting through a vertical pipe is satisfactory when the flow is continuous; when it is checked and discontinuous it is highly objectionable unless the flow is checked by baffle plates.

(b) Freezing Weather: Concrete should not be mixed or deposited at a freezing temperature, unless special precautions are taken to prevent the use of materials covered with ice crystals or containing frost, and to prevent the concrete from freezing before it has set and sufficiently hardened.

As the coarse aggregate forms the greater portion of the concrete, it is particularly important that this material be warmed to well above the freezing point.

The enclosing of a structure and the warming of a space inside the enclosure is recommended, but the use of salt to lower the freezing point is not recommended.

(c) Rubble Concrete: Where the concrete is to be deposited in massive work, its value may be improved and its cost materially reduced by the use of clean stones saturated with water, thoroughly embedded in and completely surrounded by concrete.

(d) Under Water: In placing concrete under water, it is essential to maintain still water at the place of deposit. With careful inspection the use of tremies, properly designed and operated, is a satisfactory method of placing concrete through water. The concrete should be mixed very wet (more so than is ordinarily permissible) so that it will flow readily through the tremie and into place with practically a level surface.

The coarse aggregate should be smaller than ordinarily used and never more than one inch in diameter. The use of gravel facilitates the mixing and assists the flow. The mouth of the tremie should be buried in the concrete so that it is at all times entirely sealed and the surrounding water prevented from forcing itself into the tremie. The concrete will then discharge without coming in contact with the water. The tremie should be suspended so that it can be lowered quickly when it is necessary either to choke off or to prevent too rapid flow. The lateral flow preferably should not be over 15 feet.

The flow should be continuous in order to produce a monolithic mass and to prevent the formation of laitance in the interior.

In case the flow is interrupted it is important that all laitance be removed before proceeding with the work.

In large structures it may be necessary to divide the mass of concrete into several small compartments or units to permit the continuous filling of each one. With proper care it is possible in this manner to obtain as good results under water as in the air.

A less desirable method is the use of the drop bottom bucket. Where this method is used the bottom of the bucket should be released when in contact with the surface of the place of deposit.

Concrete sewers should be constructed in longitudinal sections in a continuous operation without interruption for the entire invert, side walls, or arch. In pouring the concrete it should be kept level in the forms and should rise evenly on each side of the sewer. All rough places in the concrete should be finished smooth by brushing with a grout of neat cement and water and honeycombs should be filled with neat cement or a one-to-one mortar.

96. Sewer Brick.—The quality of brick used in sewers is seldom specified with the minute care that is taken in the specifications for concrete, iron, and certain other materials of construction, as inferior materials in brick are more easily detected. The specifications of the Baltimore Sewerage Commission for sewer brick are:

Sewer brick shall be whole, new bricks of the best quality, of uniform standard size, with straight and parallel edges and square corners: they shall be of compact texture, burned hard and entirely through, free from injurious cracks and flaws, tough and strong, and shall have a clear ring when struck together. The sides, ends and faces of all bricks shall be plane surfaces at right angles and parallel to each other. Bricks of any one make shall not vary more than 1
16th of an inch in thickness, nor more than 1⅛th of an inch in width or length, from the average of the samples submitted for approval.

The truest bricks shall be used in the face of the masonry and the exposed surfaces shall be true and smooth planes.

All bricks delivered for use shall be culled by the Contractor when required. No brick thrown out in the culling shall be used in any work done under any contract of the Sewerage Commission, except that the best of the culls may be used in manholes, above the level of the top of the sewer, if permitted by the Engineer.

The average amount of water absorbed by the bricks, after being thoroughly dried and then immersed for 24 hours, shall not exceed 6 per cent. All bricks shall be uniform in quality and percentage of absorption.

Whenever vitrified bricks are required in the invert of the sewer, they shall be smooth, hard, tough, and of such durability as will fit them for this use. They shall be of standard size, well and uniformly burned, thoroughly vitrified throughout, and free from warps, cracks, and other defects. The surfaces and edges shall be true and straight and the corners sharp and square. They shall be in every respect satisfactory to the Engineer, and in all respects equal to the sample in the office of the Engineer.

The remaining paragraphs of the specifications deal with the manner in which samples shall be submitted and the necessity for conformity between the samples submitted and the bricks used.

A common size of brick in use for sewers is 2¼ × 4 × 8¼ inches, but the variations in size are many. The bricks in use on any one job should be as near the same size as possible as the extra mortar filling necessary to make up for small brick detracts from the strength of the sewer. Small brick are undesirable as the cost of laying small and large bricks is the same, but the thickness of the finished sewer is less. Sewer brick should not absorb more than 10 to 20 per cent moisture by volume, in 24 hours; except the special paving brick used to prevent erosion at the invert which should absorb less than 5 per cent moisture.

97. Vitrified Sewer Block.—Blocks and bricks are manufactured in a manner similar to the manufacture of vitrified sewer pipe described in Art. 91. J. M. Egan describes two types of sewer blocks[^[62]] as follows:

There are on the market two designs of blocks, one being a single-ring block and the other a double-ring block. The former has a ship-lap joint on the ends and a tongue-and-groove joint on the sides. In the double block the laps and joints are made in the construction of the sewer and the blocks are placed one on top of the other as in a two ring brick sewer. The blocks are hollow longitudinally with web braces. They are made for sewers from 30 inches to 108 inches in diameter and weigh from 40 to 120 pounds. They are 18 inches to 24 inches long, 9 to 15 inches wide, and 5 to 10 inches thick. Short lengths are made for convenience in construction and for use on sharp curves. Special blocks are made for connections and junctions.

A special block is also made for inverts, which has occasionally been used with brick sewers to avoid the difficulty of constructing with brick at this point. Such blocks are objectionable, as they leave a line of weakness along the longitudinal joint so formed. They are not used frequently in present day practice.

Vitrified blocks are generally cheaper than bricks, but they do not make so strong a structure. In some cases it is possible to lay vitrified block without the expense of high-priced bricklayers, thus saving on the cost of the sewer and obtaining a conduit with a smoother interior finish.

98. Cast Iron, Steel, and Wood.—Cast iron, steel, and wood pipe belong more to the field of waterworks than of sewerage, as they are not extensively used in the construction of sewers. There are, however, some special conditions under which these materials may be serviceable.

The iron used in cast-iron pipe for sewers, and in castings for manhole covers, inlet frames, etc., is seldom carefully or definitely specified. The standard specifications of the American Water Works Association with regard to the quality of iron for water pipe are:

All pipe and special castings shall be made of cast iron of good quality and of such character as shall make the metal of the castings strong, tough, and of even grain and soft enough to satisfactorily admit of drilling and cutting. The metal shall be made without the admixture of cinder iron or other inferior metal, and shall be remelted in a cupola or air furnace.

The specifications of the Sanitary District of Chicago for the quality of iron to be used in manhole covers, etc., are given on page [101].

Although sewer pipes are not ordinarily subjected to internal pressure, cast-iron pipe for sewers should be as heavy or heavier than water pipe to resist the corrosive action of the sewage and the external stresses that are to be imposed upon it. The sizes and details of standard cast-iron pipe used for both water works and sewerage can be found in specification of the American and New England Water Works Associations.

The quality of steel used for reinforcing concrete should be carefully specified because of the possibility of the substitution of inferior material. The specifications for “Billet Steel Concrete Reinforcement Bars,” of the American Society for Testing Materials[^[63]] are the standard for engineering practice, or the following specifications may be used:

All reinforcement shall be free from excessive rust, scale, paint, or coatings of any character which will tend to destroy the bond. The bars shall be rolled from new billets. No rerolled material will be accepted. All reinforcement bars shall develop an ultimate tensile strength of not less than 70,000 pounds per square inch. The test specimen shall bend cold around a pin, whose diameter is two times the thickness of the bar, 180 degrees without cracking on the outside portion. The reinforcing bars shall in all respects fulfill the requirements of the standard specifications of the American Society for Testing Materials for Billet Steel Concrete Reinforcing Bars serial designation A 15–14.

The steel used in pipe should be a soft, open-hearth steel with an ultimate tensile strength of 60,000 pounds per square inch, an elastic limit of 30,000 pounds per square inch, an elongation in 8 inches before fracture between 22 and 25 per cent, and a reduction in area before fracture of 50 per cent. The working strength of the steel is taken at 16,000 to 20,000 pounds per square inch in tension, 10,000 to 12,000 pounds per square inch in shear, and 20,000 to 24,000 pounds per square inch in bearing. A liberal allowance should be made for corrosion. The standard specifications for Open-Hearth Boiler Plate and Rivet Steel of the American Society for Testing Materials, Aug. 16, 1919, include “flange steel,” which is suitable for the manufacture of plates, and extra soft steel which is suitable for rivets.

Steel pipe should be coated both inside and out to protect it against corrosion. The various proprietary coatings are mainly coal tar pitches, or mixtures of coal tar pitch and asphalt. A coal tar pitch is a distillate of coal tar from which the naphtha has been removed and to which about one per cent of heavy linseed oil has been added. The coating is applied to the pipe at a temperature of about 300 degrees Fahrenheit, by dipping hot pipe in the heated coating material. The pipe should be carefully cleaned and all rust and scale removed before it is dipped. In some cases the steel is pickled before dipping. This consists in rolling the cold plates to a short radius to loosen the scale, heating them to about 125 degrees, and dipping them in a warm 5 per cent acid solution for about 3 minutes, and finally rinsing in a weakly basic wash water.

The woods commonly used for the manufacture of wood pipe are spruce, Oregon fir, Douglas fir, and California redwood. Wood pipe lines have been constructed of other kinds of lumber but only in more or less unusual conditions. The following has been abstracted from the specifications for California redwood given by J. F. Partridge.[^[64]]

The staves shall be of clear, air-dried, California redwood, seasoned at least one year in the open air, and shall be free from knots (except small knots appearing on one face only), sap, dry rot, wind shakes, pitch, pitch seams, pitch pockets, or other defects which would materially impair their strength or durability. The sides of the staves shall be milled to conform to the inside and outside radii of the pipe; and the edges shall be beveled to true radial planes. The staves shall be milled from stock sizes of lumber, the net finished thickness of the stave, for the various diameters of pipe, shall be as given in Table 40. The ends shall be cut square and slotted to receive the metallic tongues which form the butt joints. The slots shall appear in the same position on each stave, and shall be cut to make a tight fit with the tongues in all directions. The staves shall have an average length of at least 15 ft. 6 in. and not more than one per cent shall have a length of less than 9 ft. 6 in. Staves shorter than 8 ft. will not be accepted.

The bands shall be spaced on the pipe with a factor of safety of at least four, and shall consist of round, mild steel rods, connected with malleable iron shoes. Either open-hearth or Bessemer steel may be used.... The ultimate strength shall be from 55,000 to 65,000 lb. per sq. in.

The original reference should be consulted for complete details and for specifications for various kinds of wood and classes of pipe. The discussion following the specifications is of value.

Machine-made wood pipe is superior to stave pipe put together in the field. It is seldom manufactured in sizes large enough for use in sewers, which results in the almost exclusive use of field constructed stave pipe. The steel bands used to hold the staves together should be coated similarly to steel plates. All lumber, except California redwood should receive a preservative coating of creosote[^[65]] or other material. One of the best methods of preserving the wood is to keep it submerged and to maintain the pipe under internal pressure.

TABLE 40
Details of Design for Continuous Stave Wood Pipe
Classes A, B, and C
(By J. F. Partridge, Trans. A. S. C. E., Vol. 82, page 461)
Diameter, Inches	Stave Thickness, Standard, Inches	Stock Size of Lumber, Inches	Size of Band, Inches	Top Width of Staves, Standard, Inches	Spacing of Bands for 100 Feet Head
12	1⅜	2 × 4	⅜	3.56	6.38
18	1–7 16	2 × 4	7 16	3.66	5.76
24	1–7 16	2 × 4	7 16	3.70	4.34
30	1½	2 × 6	½	5.48	4.53
36	1–9 16	2 × 6	½	5.62	3.77
42	1⅝	2 × 6	½	5.51	3.23
48	1⅝	2 × 6	½ or ⅝	5.60	2.84 or 4.41
60	2½	3 × 6	⅝	5.56	3.54
72	3½	4 × 6	⅝ or ¾	5.69	2.95 or 4.24
84	3½	4 × 6	¾	5.65	3.63
120	3⅝	4 × 6	¾	5.68	2.54
144	3⅝	4 × 6	¾ or ⅞	5.64	2.12 or 2.89

CHAPTER IX
DESIGN OF THE SEWER RING

99. Stresses in Buried Pipe.—The stresses which sewer pipe should be designed to resist are: internal bursting pressure, for sewers flowing under pressure; stresses due to handling, for precast pipe; temperature stresses; and external loads. The latter is by far the most important and frequently is the only stress considered in design.

The thickness of a pipe to resist internal stress should be

PR
f_t,

in which P = the intensity of internal pressure; R = the radius of the inside of the pipe, and f_t = the unit-strength of the material in tension

The derivation of this expression is simple. The stresses due to handling cannot be computed and are cared for by a thickness of material dictated by experience. These thicknesses are given for vitrified clay and cement pipe in the specifications in the preceding chapter. Temperature stresses are not allowed for in the design of the pipe ring, but allowance must be made for them in long rigid pipe lines exposed to wide variations in temperature. Such a condition seldom exists in sewerage works.

The external forces are ordinarily the controlling features in the design of sewer rings. The simplest problems arise in the design of a circular pipe. If the external loading is uniform about the circumference of the pipe the internal stresses will all be compression. Almost all other forms of loading will cause bending moments resulting in tension and compression in different parts of the pipe. The maximum bending is caused by two concentrated loads diametrically opposed. As such a condition is extreme it is not cared for in ordinary design, but a loading between this condition and perfect distribution is assumed, as explained in Art. 103.

100. Design of Steel Pipe.—The stresses which may occur in steel sewer pipes are commonly caused by the internal or bursting pressure of the contained liquid. Occasionally a steel pipe may be used as a bridge or as a stressed member of a bridge, but steel pipes should not be used to withstand compression normal to the axis. In order to avoid such stresses the bursting tensile stresses should exceed the external compressive stresses. Such a condition in design requires that buried pipes shall never be emptied, a condition that cannot always be fulfilled. Precaution should be taken, by the installation of proper valves, to prevent the emptying of the pipe at so rapid a rate that a vacuum is created resulting in the collapse of the pipe.

Steel pipes are ordinarily made of plates curved to the proper diameter, the edges being held together by rivets. The design of the pipe consists in the determination of the thickness of the plate and the design of the riveted joint. The longitudinal joint and the thickness of the plate are first designed. The design of the joint consists in determining the diameter and pitch of the rivets and the thickness of the plate so that the full strength of the uncut metal shall be developed as nearly as possible under bearing, tearing, and shearing. This is done by making the efficiency of the joint the same under all stresses. The efficiency of the joint is the ratio of the strength of the joint under any kind of stress to the strength in tension of the unpunched plate. Properties of riveted joints are given in Table 41.

The diameter of the rivet holes should be computed as 1
16 of an inch larger than the diameter of the rivets. Rivets and plates should be designed for the nearest or next largest commercial size, and a generous allowance for corrosion should be made in determining the thickness of the plate. The distance from the edge of the plate to the side of the rivet should not be less than 1½ times the diameter of the rivet. The unit-strengths of the metal are given in the preceding chapter.

The transverse joint must be designed empirically as the stresses in it are indeterminate. The common form of joint for pipes less than 48 inches in diameter is a single-riveted lap joint, and for larger pipes or for pipes exposed to unusual stresses, a double riveted lap joint is used. The same size rivets are used as in the longitudinal joint. The maximum permissible distance between rivets should be used in the transverse joint.

TABLE 41
Properties of Riveted Joints
(Chicago Bridge and Iron Works)
Type of Joint	Thickness Plate, Inch	Diameter of Rivet, Inch	Pitch, Inches	Efficiency of Joint, Per Cent	Thickness Butt Plate, Inches
Single-riveted lap	¼	⅝	1.88	49
	¼	¾	2.25	50
	5 16	⅞	2.63	50
Double riveted lap	¼	⅝	2.50	70
	5 16	¾	3.00	71
	⅜	⅞	3.40	71
Triple riveted lap	¼	½	2.39	74
	5 16	⅝	2.96	74
	⅜	¾	3.53	75
	7 16	⅞	4.09	76
Quadruple riveted lap	⅜	⅝	3.20	77
	7 16	¾	3.90	78
Double riveted butt	½	⅞	3.62	72	⅜
	9 16	⅞	3.62	72	⅜
	⅝	⅞	3.62	72	⅜
	11 16	⅞	3.62	72	7 16
	¾	1	4.12	73	7 16
	⅞	1	3.82	71	½
	1	1	3.48	68	9 16
Triple riveted butt	⅝	⅞	4.94	80	½
	¾	1	5.62	80	9 16
	⅞	1	5.16	78	9 16
	1	1	4.66	76	9 16
Quadruple riveted butt	¾	1	7.13	84	¾
	⅞	1	6.51	83	11 16
	1	1	5.84	81	⅝

Pipes used as compression members of a bridge are stiffened by riveting standard rolled steel sections longitudinally on the pipe.

Fig. 78.—Lock Bar Pipe.

Lock Bar Pipe is a steel pipe with a special form of joint made by the East Jersey Pipe Corporation. It is arranged as shown in Fig. 78 and has the advantage of developing the full strength of the plate. It is equivalent to a joint with 100 per cent efficiency, which permits the use of thinner plates.

101. Design of Wood Stave Pipe.—In the design of wood stave pipe[^[66]] the entire bursting pressure is taken up by steel bands wrapped around the outside of wood staves which make up the shell of the pipe. The pipe is not designed to resist external loads except those which may be overcome by the internal pressure in the pipe. The thickness of the staves is fixed by experience. The sizes of staves and bands recommended by J. F. Partridge[^[67]] are given in Table 40. The size of the steel bands can be determined from the expression;

S = Cr(R + t)

in which S = the total stress in the band; R = the radius of the inside of the pipe; t = the thickness of the stave; r = the area of bearing per unit length of the band on the wood. For circular bands it is assumed as the radius of the band; C = the crushing strength of wood, usually taken at 650 pounds per sq. in.

The preceding expression can be derived easily by the application of the laws of mechanics, and from it the expression for the distance between bands follows logically. It is,

p = S
PR + kt

in which S = the strength of the band; p = the distance between bands; P = the intensity of bursting pressure in the pipe; R = the radius of the inside of the pipe; t = the thickness of the staves; k = the swelling strength of wood, usually taken at 100 pounds per sq. in.

Fig. 79.—Shoe for Wood Stave Pipe.

Transverse joints between staves are closed by inserting metal strips between them, or by shaping the edges irregularly so that they fit closely together with an irregular joint. Transverse joints between all staves at any one point are avoided by splitting the joints between staves. Longitudinal joints between staves are usually made smooth and are closed by steel bands which are drawn tight about the pipe by inserting the ends in coupling shoes as shown in Fig. 79.

Fig. 80.—B in Formula W = CwB²

102. External Loads on Buried Pipe.—Prof. Anston Marston and H. C. Anderson published[^[68]] the results of a series of experiments on the loads on buried pipes which are of extreme value in the design of sewer pipe. The load on the pipe is given by the empirical expression W = CwB², in which w is the weight of the backfilling material in pounds per cubic foot, B is the width of the trench in feet at the elevation of the end of a radius making an angle of 45 degrees upwards with the horizontal diameter of the pipe as illustrated in Fig. 80, and C is a coefficient dependent on the character of the backfill and the ratio of the width to the depth of the trench. Values of C are given in Table 42. The weights of various classes of backfilling are given in Table 43.

TABLE 42
Approximate Safe Working Values of C in the Expression W = CwB2
From Bulletin No. 31 of the Engineering Experiment Station, Iowa State College of Agriculture.
Ratio of Depth to Width	Approximate Values of C
	Damp Top Soil and Dry and Wet Sand	Saturated Top Soil	Damp Yellow Clay	Saturated Yellow Clay
0.5	0.46	0.47	0.47	0.48
1.0	0.35	0.86	0.88	0.90
1.6	1.16	1.21	1.25	1.27
3.0	1.47	1.51	1.56	1.62
2.6	1.70	1.77	1.83	1.91
3.0	1.90	1.99	2.08	2.19
3.6	2.08	2.18	2.28	2.43
4.0	2.22	2.35	2.47	2.65
4.6	2.34	2.49	2.63	2.85
6.0	2.45	2.61	2.78	3.02
6.5	2.54	2.72	2.90	3.18
6.0	2.61	2.81	3.01	3.32
6.6	2.68	2.89	3.11	3.44
7.0	2.73	2.95	3.19	3.55
7.5	2.78	3.01	3.27	3.66
8.0	2.82	3.06	3.33	3.74
8.5	2.85	3.10	3.39	3.82
9.0	2.88	3.14	3.44	3.89
9.5	2.90	3.18	3.48	3.96
10.0	2.92	3.20	3.52	4.01
11.0	2.95	3.25	3.58	4.11
12.0	2.97	3.28	3.63	4.19
13.0	2.99	3.31	3.67	4.25
14.0	3.00	3.33	3.70	4.30
15.0	3.01	3.34	3.72	4.34
∞	3.03	3.38	3.79	4.50

TABLE 43
Approximate Weights of Ditch Filling Material to be Used in the Expression W = CwB2[[69]]
Ditch Filling	Pounds per Cubic Foot
Partly compacted top soil (damp)	90
Saturated top soil	110
Partly compacted damp yellow clay	100
Saturated yellow clay	130
Dry sand	100
Wet sand	120

Where surface loads are to be carried on the sewer trench the proper proportion of the load to be carried by the sewer is determined by the expression L_p = CL, in which L_p is the equivalent backfill load per unit length of the trench, L is the surface load per unit length of the trench, and C is a coefficient in which allowance is made for the character of the backfilling, the ratio of depth to width of trench, and the character of the load, whether long or short. A long load is a load extending along the length of the trench such as a pile of building material. A short load is one extending across the trench and for only a short distance along it, such as that caused by a street car or road roller crossing the trench. Values of C are given in Table 44 for long loads, and in Table 45 for short loads. Values of long and short loads occasionally met in practice are given in Tables 46 and 47 respectively.

TABLE 44
Ratio of Load on Pipe to Long Load on Trench[[70]]
Ratio of Depth to Width	Sand and Damp Top Soil	Saturated Top Soil	Damp Yellow Clay	Saturated Yellow Clay
0.0	1.00	1.00	1.00	1.00
0.5	0.85	0.86	0.88	0.89
1.0	0.72	0.75	0.77	0.80
1.5	0.61	0.64	0.67	0.72
2.0	0.52	0.53	0.59	0.64
2.5	0.44	0.48	0.52	0.57
3.0	0.37	0.41	0.45	0.51
4.0	0.27	0.31	0.35	0.41
5.0	0.19	0.23	0.27	0.33
6.0	0.14	0.17	0.20	0.26
8.0	0.07	0.09	0.12	0.17
10.0	0.04	0.05	0.07	0.11

For example, let it be desired to determine the load on a 72–inch concrete sewer with a 9–inch shell under the following conditions: depth of backfill over the top of the pipe, 15 feet; character of backfill, saturated yellow clay; superimposed load, pile of building brick 6 feet high. The ratio of the depth of backfill to the width of the trench is 15 ÷ 9 or 1.67. The coefficient in the expression CwB² is 1.39, from Table 42. The weight of saturated yellow clay is 130 pounds per cubic foot, from Table 43. Therefore the load per foot length of the sewer due to the backfill is:

W = CwB² = 1.39 × 130 × 81 = 14,600 pounds.

TABLE 45
Ratio of Load on Pipe to Short Load on Trench[[71]]
Ratio of Height to Width of Trench	Sand and Damp Top Soil		Saturated Top Soil		Damp Yellow Clay		Saturated Yellow Clay
	Length of Load Equal to
	Width of Trench	⅒ Width of Trench	Width of Trench	⅒ Width of Trench	Width of Trench	⅒ Width of Trench	Width of Trench	⅒ Width of Trench
0.0	1.00	1.00	1.00	1.00	1.00	1.00	1.00	1.00
0.5	0.77	0.12	0.78	0.13	0.79	0.13	0.81	0.13
1.0	0.59	0.02	0.61	0.02	0.63	0.02	0.66	0.02
1.5	0.46		0.48		0.51		0.54
2.0	0.35		0.38		0.40		0.44
2.5	0.27		0.29		0.32		0.35
3.0	0.21		0.23		0.25		0.29
4.0	0 12		0.12		0.16		0.19
5.0	0.07		0.09		0.10		0.13
6.0	0.04		0.05		0.06		0.08
8.0	0.02		0.02		0.03		0.04
10.0	0.01		0.01		0.01		0.02

TABLE 46
Weights or Common Building Material When Piled for Storage. Pounds per Cubic Foot
Brick	120
Cement	90
Sand	90
Broken stone	150
Lumber	35
Granite paving	160
Coal	50
Pig iron	400

The pressure of the pile of brick per square foot of trench area is, from Table 46, 120 × 6 = 720 pounds per square foot. The value of C from Table 44, is about 0.70. Therefore L_p is 0.7 × 9 × 720 = 4536 pounds. The equivalent depth of backfill weighing 130 pounds per cubic foot is 4536
130 × 9 = 3.88 foot. The total equivalent depth of backfill is therefore 3.88 + 15 = 18.88 feet. The ratio of depth to width is 18.88
9 = 2.98. The coefficient C in the expression W = CwB² is 2.17. The total load per foot length of sewer is therefore W = 2.17 × 130 × 81 = 22,800 pounds.

TABLE 47
Weights of Short Loads on Sewer Trenches
(Adapted from Specifications of the American Bridge Company for Bridges)
Street railways, heavy	A load of 24 tons on 2 axles on 10 foot centers.
Street railways, light	A load of 18 tons on 2 axles on 10 foot centers.
For city streets, heavy traffic	A load of 24 tons on 2 axles 10 feet apart and 5 foot gage.
For city streets, moderate traffic	A load of 12 tons on 2 axles 10 feet apart and 5 foot gage.
For city streets, light traffic or country roads	A load of 6 tons on 2 axles 10 feet apart and 5 foot gage.
Road rollers	Total weight 30,000 pounds. Weight on front wheel, 12,000 pounds, and on each of two rear wheels, 9,000 pounds. Width of front wheel, 4 feet and of each of two rear wheels 20 inches. Distance between front and rear axles 11 feet. Gage of rear wheels, 5 feet, c. to c.

103. Stresses in Circular Ring—In Fig. 81a the loads shown indicate the distribution ordinarily assumed in sewer design, the forces being uniformly distributed across the diameter. To find the bending moment in the pipe caused by this loading, let ab in Fig. 81b represent a section of a pipe loaded with equally distributed horizontal and vertical forces. Then the vertical component on a strip of differential length ds is wds cos Θ and the horizontal component is wds sin Θ and resolving, the resultant normal to the surface is wds, in which w is the intensity per unit length of the horizontal and vertical forces and Θ is the angle which the tangent to ds makes with the horizontal. Thus the loading of the nature shown in Fig. 81b is equivalent to a loading of equally distributed normal forces which give no moment in the ring.

Fig. 81.—Distribution of Stresses on Buried Pipe.

Considering a ring subjected to vertical forces only, the moments will be as shown in Fig. 81c and if loaded with horizontal forces only, the moments will be as shown in Fig. 81d. Because of the symmetry of the figure, moment (1) equals moment (4) but is opposite in direction and moment (2) equals moment (3) but is opposite in direction. When the horizontal and vertical forces are combined on the same ring as in Fig. 81b these moments cancel each other as has been proven. Therefore moment (1) equals moment (2) and moment (3) equals moment (4). Then in Fig. 81e, M_a = M_b. Now ∑M = O for conditions of equilibrium, therefore M_a + M_b + (W
2)(d
4) = O and solving M_a = Wd
16. This moment occurs at the ends of the horizontal and vertical diameters and causes tension on the inside of the pipe at the top and on the outside at the ends of the horizontal diameter. There will also be compression at each end of the horizontal diameter equal to one-half of the total load on the pipe. If the material of the pipe is homogeneous, the maximum fiber stress f can be found through the expression f = My
I ± P
A in which M is the bending moment, y is the distance from the neutral axis to the extreme fiber of a cross-section of the shell of the pipe of unit length, I is the moment of inertia of this cross-section about its neutral axis, P is one-half the total load on the pipe, and A is the area of the cross-section. For reinforced concrete, the standard formulas should be used with this expression for M. The stresses in a circular ring subjected to other distributions of loads are shown in Table 48. An exhaustive study of the stresses in circular rings was published by Prof. A. N. Talbot in Bulletin No. 22 of the Engineering Experiment Station at the University of Illinois, 1908.

TABLE 48
Maximum Stress in Flexible Rings Due to Different Loadings
(From Marston)
Symmetrical Vertical Loadings		Moment at Crown of Sewer	Moment at End of Horizontal Diameter	Compressive Thrust at Crown	Compressive Thrust at End of Horizontal Diameter	Shear at Crown	Shear at End of Horizontal Diameter
Character	Width
Concentrated	0°	+ .318RW 12	- .182RW 12	0.000	+ .500W 12	0.500W 12	0.000
Uniform	60°	+ .207RW 12	- .168RW 12	0.000	+ .500W 12	0.000W 12	0.000
Uniform	90°	+ .169RW 12	- .154RW 12	0.000	+ .500W 12	0.000W 12	0.000
Uniform	180°	+ .125RW 12	- .125RW 12	0.000	+ .500W 12	0.000W 12	0.000
R = the radius of the pipe, W = total weight of ditch filling and superimposed load plus ⅝ of the weight of the pipe itself (usually neglected), expressed in pounds per foot length of pipe. Moments are inch-pounds per inch length of pipe. Shears and thrusts are in pounds per inch length of pipe.

104. Analysis of Sewer Arches.—The preceding method for the determination of the stresses in a sewer ring has referred only to a circular pipe uniformly loaded. Other methods must be used if the pipe is not circular or the load is not uniformly distributed. The simplest method, is the static or so-called vouissoir method. In this method the arch is assumed to be fixed at both ends, presumably at the springing line or line of intersection between the inside face of the arch and the abutment, and it is so designed that the resultant of all the forces acting on any section shall lie within the middle third of that section.

Fig. 82.—Voussoir Arch Analysis.

Fig. 83.—Force Polygon for Voussoir Arch Analysis.

To design an unreinforced sewer arch by the vouissoir method, a desired arch is drawn to scale in apparently good proportions for the loadings anticipated. The arch is then divided into any number of sections of equal or approximately equal length called vouissoirs, and the line of action of the resultant load, including the weight of the vouissoir is drawn above each vouissoir as shown in Fig. 82. The forces are assumed to act as shown in the figure. In symmetrically loaded sewer arches there is no vertical reaction at the crown. The resultant R is assumed to act at the lower middle third of the skewback, which is the inclined joint between the arch and the abutment. The upper horizontal force H is assumed to act at the upper middle third of the middle or crown section. The magnitude of H is computed by equating the sum of the moments of all forces about the point of application of R at the skewback to zero, and solving. The force polygon is then drawn as shown in Fig. 83, and the equilibrium polygon is completed in Fig. 82 with its rays parallel to the corresponding strings drawn from the end of H as origin in Fig. 83. If the equilibrium polygon line, called the resistance line, lies wholly within the middle third of each vouissoir, the arch is satisfactory to support the assumed load without reinforcement. If any portion of the resistance line lies outside of the middle third, an attempt should be made to find a resistance line which lies wholly within the middle third. The true resistance line is that which deviates the least from the neutral axis of the arch. To approximate more nearly the true resistance line find two points at which the resistance line already drawn deviates the most from the neutral axis of the arch. Select points M and N on these joints, M being nearer the crown than N. Then let W₁ and W₂ be the sum of all the loads between the crown and M and N respectively, y represent the vertical distance from the crown to N, and y′ represent the vertical distance between M and N, and x₁ and x₂ represent the horizontal distance from W₁ and W₂ to M and N respectively. Then the horizontal thrust, H, and a, the distance from the crown to the point of application of H, are,

H = (W₂x₂ − W₁x₁)
y′,

a = y − W₂x₂
H.[^[72]]

A resistance line should be drawn with this new horizontal thrust. If no resistance line can be found lying wholly within the middle third, new sections should be designed until a resistance line can be drawn lying wholly within the middle third—unless the arch is to be reinforced. A number of satisfactory arches should be designed and the easiest one to build should be selected. This method is limited in its application to sewer arches with rigid side walls and it cannot be extended to include the invert. Although an approximate method it is accurate within less than 10 per cent of the true stresses and is usually quite close.

Fig. 84.—Method for Dividing Arch into Proportion I
S.

The elastic method for the design of arches locates the true line of resistance without approximations and is more accurate though not so simple to apply as the static or vouissoir method. In this method a desired form of arch is drawn as in the static method and subdivided into vouissoirs so that the distance S along the neutral axis between joints is such that the ratio I
S shall be the same for all vouissoirs. I is the average of the moments of inertia of the surfaces of the two limiting joints about the neutral axis. If the thickness of the arch is constant the distance between joints will be the same. The method for dividing the arch into sections such that the ratio I
S shall be a constant[^[73]] is as follows: divide the half arch axis into any number of equal parts; measure the radial depth at each point of division; lay off the length of the arch axis to scale on a straight line; divide this line into the same number of equal parts as the half arch, as shown in Fig. 84; at each point erect a perpendicular equal in length by scale to the moment of inertia at the corresponding point on the arch section; draw a smooth curve through the tops of these lines; draw a line ab at any slope from the center of the original straight line to the curve, and then a line bc back to the straight line to form an isosceles triangle abc; continue forming these triangles in a similar manner thus dividing the original straight line in the required ratio. The distance between joints is represented by the bases of the triangles. By construction the altitude of the triangle represents the average moment of inertia between the two limiting joints. The base of each isosceles triangle is S, and I
S = ½ tan α in which α is the base angle of all the isosceles triangles.

Fig. 85.—Elastic Arch Analysis.

The following steps in the procedure are taken from the second edition of the American Civil Engineers Pocket Book, p. 634:

In Fig. 85 let the middle points of the joints be marked 1, 2, 3, etc. and the coordinates x and y from the crown be found for each by computation or measurement. For a load W placed at one of these points, let z denote the distance from it, toward the nearest skewback, to another middle point. Let ∑zx be the sum of the products of all the values of z by the corresponding x, and ∑zy be the sum of all the products of z by the corresponding y; that is, each z in the last two summations is multiplied by the x or y of the point back of W which corresponds to z.

For a single load W on the left semi-arch of Fig. 85 the following formulas are deduced from the elastic theory, n being the number of parts into which the semi-arch is divided.

Horizontal thrust, H = (W
2)n∑zy − ∑y·∑z
n∑y² − (∑y)² (1)

Moment at Crown, M₀ = ½W∑z − H∑y
n (2)

Shear at Crown, V₀ = ½W∑zx
∑x² (3)

For symmetrical loading such as W on the left and W on the right the horizontal thrust and crown moment due to both loads are double those found by the above formulas, while the crown shear V₀ is zero. For several loads unsymmetrically placed the formulas are to be applied to each in succession and the results added algebraically, the value of V₀ being taken as negative for the left semi-arch and positive for the right semi-arch.

For any joint whose middle point is at a distance x from the crown

M = M₀ + Hy + V₀x − ∑Wz,

V = V₀ − ∑W,

where ∑W is the sum of all the loads between the joint and the crown and ∑Wz is the sum of the moments of those loads with respect to the middle of the joint. The components of the resultant thrust normal and parallel to the joints are,

N = H cos θ − V sin θ,

F = H sin θ + V cos θ,

in which θ is the angle which the plane of the joint makes with the vertical.

The distances from the neutral axis to the resistance line are,

at the crown, e₀ = M₀
H,

at the joint, e = M
N.

The resistance line should be located as in the vouissoir method and if not within the middle third a new design should be studied.

105. Reinforced Concrete Sewer Design.—The method to be followed in the design of reinforced concrete arches is similar except that the moment of inertia should include both the concrete and the steel, that is,

I = I_c + nI_s,

in which I is the moment of inertia to be employed, I_c is the moment of inertia of the concrete, I_s is the moment of inertia of the steel, and n is the ratio of their moduli of elasticity, generally taken as 15. All of the moments of inertia are referred to the neutral axis of the beam. The reinforcement called for in precast circular pipes is given in Table 39. Sewers cast in place are ordinarily designed to avoid reinforcement, except where the depth of cover is small and the sewer may be subjected to superimposed loads.

Concrete sewers are sometimes reinforced longitudinally, with expansion joints from 30 to 50 feet apart. This reinforcement is to reduce the size of expansion and contraction cracks by distributing them over the length of a section. The pipe is divided into sections to concentrate motion due to expansion or contraction at definite points where it can be cared for.

The amount of longitudinal reinforcement to be used is a matter of judgment. It varies in practice from 0.1 to 0.4 per cent of the area of the section. Since the coefficients of expansion of concrete and of steel are nearly the same, movements of the structure are as important as the stresses due to changes in temperature.

Because of the uncertain and difficult conditions under which concrete sewers are frequently constructed it is advisable to specify the best grade of concrete and not to stress the concrete over 450 pounds per square inch in compression, with no allowable stress in tension. The concrete covering of reinforcing steel should be thicker than is ordinarily used for concrete building design, because of the possibility of poor concrete allowing the sewage to gain access to the steel, resulting in more rapid deterioration than would be caused by exposure to the atmosphere. A minimum covering of about 2 inches is advisable, except in very thin sections not in contact with the sewage. A minimum thickness of concrete of about 9 inches is frequently used in design, although crown thicknesses of 4½ inches have been used with success. Greater thicknesses should be used near the surface, particularly in locations subjected to heavy or moving loads.

Brick linings are often provided for the invert where moderately high velocities of about 10 feet per second when flowing full are to be expected. For velocities in the neighborhood of 20 feet per second the invert should be lined with the best quality vitrified brick. Although concrete may erode no faster than brick under the same conditions, brick linings are more easily replaced and at a smaller expense.

CHAPTER X
CONTRACTS AND SPECIFICATIONS

106. Importance of the Subject.—Sewers may be constructed by day labor or by contract. Under the day labor plan a city official or commission is charged with the purchase of material, the hiring and firing of employees, and the management of the work. Under the contract system a private individual or company contracts to supply all the material and labor necessary for the completion of the work.

Under the day labor plan all persons engaged are “working for the City.” There is not the same sense of individual responsibility, the same incentive to economize, the same feeling of loyalty that is inspired by work under the personality of a contractor. Under either the day labor or contract plan unscrupulous politics are likely to enter into the relations of the employees of the city and the city officials or between the contractor and the city officials. Neither the day labor nor the contract plan offer a sure cure for unscrupulous political misdealings. Under the contract plan the contractor is led to keep his bid as low as possible, realizing the competition of other bidders, and during construction he will obtain greater efficiency from his labor because of their realization of the different conditions under which they are working. In some states and cities it is illegal for the municipality to do sewer construction except under the contract method.

The contract method is therefore used in the majority of cases, and it is to the interest of the engineer that he be acquainted with the essentials of contracts and specifications necessary for the proper prosecution of sewer construction.

107. Scope of Subject.—The making of a contract is one of the most common episodes of every day life. The contract may be an informal verbal agreement to meet at a certain place at a certain time, or it may be a formal document hedged about by confusing legal phraseology and bearing varieties of penalties and dire consequences in the event of its breach. The purpose of this chapter is to explain only those general features of an engineering contract which have particular bearing upon sewerage construction. Only the most essential points can be touched in the limited space available to this subject, it being presumed that the engineer is previously grounded in the principles of business law.[^[74]]

108. Types of Contracts.—Contracts are known as lump sum, cost-plus, unit-price, and by other titles indicating the method of payment.

A lump sum contract is one in which a stated amount is fixed upon, before the execution of the contract, to be paid for all the work to be done and materials to be furnished under the contract. Such an arrangement is not advisable for a sewer contract, as the cautious contractor will bid high enough to protect himself in the event of any probable emergency. The principal must therefore pay whether the emergency or unforeseen difficulty is met or not. The advantage of this type of payment is that the principal knows exactly the cost of the work to him before construction is commenced.

Cost-plus contracts are those in which the cost of the work to the contractor is to be paid by the principal, plus, (a) a fixed sum of money, (b) a percentage of the cost of the work, (c) a percentage of the cost of the work but with a fixed limit, (d) a percentage of the difference between the cost of the work and some fixed sum, or other variations of this principle. Such contracts have the advantage that the principal assumes all the risk in construction and therefore pays for only those contingencies which actually arise. Except for the last named form, they have the disadvantage that there is little or no incentive for the contractor to keep the cost of the work down. They are most successful where the contractor can be selected by the principal, but where it is necessary to let contracts to the lowest bidder, the “cost-plus” contract is not easily managed. In most states a municipality cannot make a cost-plus contract.

A unit-price contract is one in which the amount to be paid is fixed in proportion to the amount of work done or materials supplied. This type of contract is the most suitable for sewer construction for a municipality where the contract must be let to the lowest bidder. The contractor is protected in the event of many unforeseen emergencies and the principal is protected against a raise in bids to cover such emergencies and against increase in the cost of the work in order to increase the profits under a “cost-plus” contract.

It is sometimes desirable for the principal to furnish a portion of the materials, the bidders being notified beforehand that this material will be furnished. In this manner the quality of material is assured, contractors with the necessary skill but small capital may be attracted to bid, and uncertainties in the procuring of materials is eliminated.

109. The Agreement.—A contract is an agreement between two or more interested parties to do a certain thing. A contract for the construction of a sewer is an agreement between a municipality or individual desiring sewerage facilities and a company or individual engaged in the construction of sewers. The latter promises to construct a sewer in return for which the former promises to pay a certain amount of money.

The various portions of the agreement which are bound together as the complete contract are: I. The Advertisement, II. Information and Instructions for Bidders, III. Proposal, IV. General Specifications, V. Technical Specifications, VI. Special Specifications, VII. Contract, VIII. Bond, and IX. Contract Drawings. These should be fastened together in pamphlet form and constitute the complete instrument called the contract. No binding contract and specifications can be drawn upon logical deductions alone as legal precedent and tried methods must be followed to insure success. To draw up an original contract requires the combined knowledge of an engineer and a lawyer. The engineer of to-day writes his specifications by copying copiously from specifications used on work which has been completed successfully. In order that selections may be made with judgment and discrimination some examples have been selected from existing published specifications and contracts.

110. The Advertisement.—This should contain: (1) A heading indicating the type of work, (2) A statement as to when, where and how bids will be received and opened, (3) A brief description of the character and amount of work to be done, (4) The method of payment, (5) The conditions under which further information can be obtained, (6) A statement as to the amount of money which must be deposited with the bid, and (7) Any other pertinent facts concerning the work.[^[75]] An example of an advertisement follows;

Sewer Construction

Construction Turkey Creek Sewer

Kansas City, Missouri.

Bids for the construction of the Turkey Creek Sewer, two sewage pumping stations to be used in connection therewith, and certain laterals and extensions of existing sewers thereto, for Kansas City, Missouri, will be received up to 2 p.m. August 19, 1919, at the office of the Board of Public Works, City Hall, Kansas City, Missouri.

The main sewer will be about one and one-fifth miles long, and the laterals and extensions about three and one-half miles: the main sewer will be constructed of reinforced concrete, the laterals and extensions will consist of concrete, segment blocks, and clay pipe.

This work is estimated to cost from $1,500,000 to $1,750,000. Payment for the work will be made in four year special tax bills, bearing 7 per cent interest, payable one-fourth each year. Time 600 working days, barring strikes, bad weather, etc.

Bidders are required to deposit $15,000 in cash or a certified check with bid, to insure signing of contract when let. Same to be returned on execution of the contract or rejection of bid.

Complete plans and specifications for the work may be had and all information obtained by seeing or writing to A. D. Ludlow, Engineer of Sewers, City Hall, Kansas City, Missouri. Twenty-five ($25.00) Dollars will be required to be deposited for a set of the plans, but $20.00 thereof will be refunded upon return of the plans in good condition.

BOARD OF PUBLIC WORKS,

Kansas City, Missouri,

by F. E. McCabe, Secretary.

There are usually legal restrictions which require that the advertisement be inserted a certain number of times in specified newspapers or other advertising mediums before the opening of bids. If the contract is of sufficient size to attract outside contractors, the advertisement should be inserted in engineering and contracting journals of wide circulation. Although the advertisement appears separately from the other portions of the contract, a copy is usually bound in as the first page of the pamphlet containing the contract and specifications and is made an integral part thereof.

111. Information and Instructions for Bidders.—This is somewhat on the order of an introduction to the pamphlet in which the specifications, contract, and contract drawings are published. As examples of the type of information and instructions given to prospective bidders the abstracts below have been taken from the “Contract, Specifications, Bond, and Proposal for the North Shore Sanitary Intercepting Sewer” by the Sanitary District of Chicago. The information and instructions to bidders can be divided into the following sections: 1st. Examination of Site, 2nd. Character and Quantity of Work, 3rd. Qualification for Bidding, 4th. Instructions for Making out Proposal, 5th. Certified Check, and 6th. Rejection of Bids.

Requirements for Bidding and Instructions To Bidders

Bidders are required to submit their bids upon the following express conditions:

Bidders must carefully examine the entire sites of the work and the adjacent premises, and the various means of approach to the sites, and shall make all necessary investigations to inform themselves thoroughly as to the facilities for delivering and handling materials at the sites and to inform themselves thoroughly as to all the difficulties that may be involved in the complete execution of all work under the attached contract in accordance with the specifications hereto attached.

Bidders are also required to examine all maps, plans, and data mentioned in the specifications, contract or proposal as being on file in the office of the Chief Engineer, for examination by bidders. No plea of ignorance of conditions that exist or that may hereafter exist or of conditions or difficulties that may be encountered in the execution of the work under this contract, as the result of a failure to make the necessary examinations and investigations, will be accepted as an excuse for any failure or omission on the part of the Contractor to fulfill in every detail all of the requirements of said contract, specifications and plans, or will be accepted as a basis for any claims whatsoever for extra compensation. Upon application all information in the possession of the Chief Engineer will be shown to bidders, but the correctness of such information will not be guaranteed by the Sanitary District.

The following schedule of quantities, although stated with as much accuracy as is possible in advance, is approximate only, and is assumed solely for the purpose of comparing bids.

Then follows an itemized schedule of the quantity of work to be done after which comes the following:

Bidders must determine for themselves the quantities of work that will be required, by such means as they may prefer, and shall assume all risks as to variations in the quantities of the different classes of work actually furnished under the contract. Bidders shall not at any time after the submission of this proposal, dispute or complain of the aforesaid schedules of quantities or assert that there was any misunderstanding in regard to the amount or the character of the work to be done, and shall not make any claims for damages or for loss of profits because of a difference between the quantities of the various classes of work assumed for comparison of bids and the quantities of work actually performed.

Proposals that contain any omissions, erasures, or alterations, conditions or items not called for in the contract and plans attached hereto, or that contain irregularities of any kind, will be rejected as informal.

Bids manifestly unbalanced will not be considered in awarding the contract.[^[76]]

No bid will be accepted unless the party making it shall furnish evidence satisfactory to the Board of Trustees of the Sanitary District of Chicago of his experience and familiarity with work of the character specified and of his financial ability to successfully and properly prosecute the proposed work to completion within the specified time.

Each bid shall be accompanied by a certified check, or cash, to the amount of ten (10) per cent of the total amount of said bid figured on the quantities given herewith, the lowest alternative total being allowed. Said amounts deposited with bids, shall be held until all of the bids have been canvassed and the contract awarded and signed. The return of said check or cash to the bidder to whom the contract for said work is awarded will be conditioned upon his appearing and executing a contract for the work so awarded and giving bond satisfactory to said Board of Trustees, for the fulfillment of each contract in the amount of fifty (50) per cent of the amount of each contract.

The said Board of Trustees reserves the right to reject any or all bids.

Accompanying the contract form are plans which, together with the specifications, show the work on which said tenders are to be made.

The proposal must not be detached herefrom or from the contract by any bidder when submitting a bid.

112. Proposal.—The proposal is a blank printed form on which the bidder is required to enter the prices for which he proposes to do the work. The proposal blank is necessary in order that the bids may be sufficiently uniform for proper comparison. Sewers are often paid for, particularly for small sizes, per foot of completed sewer as measured along the center line of the pipe parallel to the surface of the ground with the exterior length of manholes and other structures deducted. Sometimes, under other conditions, a different rate is allowed for each additional two feet of depth of sewer, and special structures, such as manholes, catch-basins, flush-tanks, etc., are paid for at a unit price according to the depth. Water connections to flush-tanks are paid for per foot of length of service pipe laid. In especially large or difficult work, materials are paid for at a unit-price, for example, per cubic yard of excavation, per cubic yard of concrete, per thousand feet board measure of lumber, etc.

The following example is taken from the contract for the North Shore Intercepting Sewer previously quoted, to indicate the type of Proposal used: